How to Give Advice to a Friend Without Being a Know-It-All

Relationship and family therapist Roger S. Gil to get some advice for dealing with these situations.
First Off: Make Sure They Want Your Advice
Obviously not everyone is looking for your advice. Before you go offering up your point of view, make sure they're interested in hearing it. As Roger Gil points out, oftentimes friends aren't looking for you to solve a problem. They just want you to listen and maybe ask some questions. Gil explains why:
People often think that therapists spend most of their time giving advice. The truth is that we usually listen and ask questions that get people to take a hard look at what rationale informs their opinions, how they feel about various things, what they have tried in the past, etc. In other words, we make the person go through the process of developing their own solutions rather than listing a series of steps to take. Most of the time a person that's seeking a solution will be asking questions about topics that don't have clearly defined "best practices" or steps that one should take.
So, ask if they'd like to hear your input or insights on a problem, but also ask questions about why they feel a certain way. If they say "no," let them finish their story and listen politely. Gil also adds that even when you know the answer to something, you might want to keep your mouth shut:
Research has shown that men stereotypically try to solve problems as quickly as possible when their mate confides in them; however this often leads to conflict because the confiding party feels "unheard." Sometimes a person just needs to vent and isn't necessarily seeking advice. Even if you know the answer, advice is often better-received once a person has shared some feelings.
If nothing else, wait for them to finish venting before offering advice (or asking if they want it). Sometimes the best way to figure something out is to do it on your own.
Differentiate Between Opinions, Expert Advice, and Being a Sounding Board
Speaking of listening, it's also a good idea to figure out right away what your friends want from from you. Gil describes this as differentiating between opinions, expert advice, and being a "sounding board:"
Different situations require different approaches so we need to know what we should contribute. Opinions are good for subjective inquiries (e.g. Should I buy a MacBook or PC?). Expert advice should be limited to areas that you KNOW very well and can offer well-informed opinions (e.g. What makes for a good SSD for a MacBook?). Sometimes a person just needs us to listen and clarify things for them (e.g. So you're saying that you're considering a MacBook because your iMac doesn't fit in your backpack). Whichever approach you take, make sure you don't present one type as another (i.e. don't present an opinion as expert advice).
Essentially, don't pretend like you know something you don't, and definitely don't act like you've been in a situation you haven't been in.
Minimize "You should.." Statements
Now that you know whether or not your friends or family actually want your advice, it's time to learn how to deal it out without sounding like a know-it-all. This is a lot harder than you might think. As Gil points out, one way to offer advice without sounding pretentious is to avoid "You should" statements:
"You should..." statements can come off as pretentious and judgmental at times. They can also make us responsible for any negative outcomes the advice seeker experiences. Using "I feel..." statements shares your idea while conveying the message that it's just the way you feel and up to the other party to take it as advice for a course of action. Example: Don't say, "You should dump his cheating self." Do say, "When I hear you talking about his cheating, I feel like staying with him could lead to more headaches for you."
Gil's advice seems obvious, but it's incredibly easy to get on your high-horse and use "You should" if you're not careful. By offering up your opinion clearly defined as your opinion, you remove the insinuation that "you know best."
Accept That You Might Have to be a Jerk
Sometimes you do need to offer up some tough love. While you should still follow the practices mentioned above, when a situation warrants it, don't be afraid to be a bit of a jerk. Gil explains:
There are no "best practices" when talking about difficult topics and sometimes one HAS to come across as a bit of a jerk to get a point across to someone who is in denial or going in circles with their conversation.
Gil points out that this list isn't exhaustive, and every situation is a bit different. However, the key is to make sure you stay in "listening mode" for as long as possible, and you don't push your advice when it isn't wanted. If you approach it right, you should be able to help your friends or family without coming off as a know-it-all.

How Do I Get Out of an Argument with an Irrational Person?

Step 1: Make Sure You Know What You're Talking About
First, check yourself —make sure you're not the one being irrational here, and you're not the one using faulty information to prove your point and make sure that your positions are well thought out, researched, and backed up, and that you're approaching the discussion calmly. When presented with an opposing view, take a little time to research it and make sure you're not the one reacting poorly.
You note that this tends to happen with this person frequently, so it's possible you're walking into a debate with him knowing that things will likely turn badly. Whether you're being baited into an argument or your mind is just already set on a fierce debate when you speak up, you need to make sure that your goal is to make your case intelligently and with an informed perspective, not convert him or force him to back down. The former may not end with the other party admitting defeat, but the latter will almost certainly never happen.
Step 2: Learn When to Fold, Even If You're Right
When you get into a heated discussion with someone, the first thing you need to think about, probably before you really go to town with your argument, is how far you're willing to let this discussion go. If it continues to escalate and escalate and it's absolutely clear that no minds are about to be changed over the course of the conversation, or if you realize that this has changed from a civil exchange of ideas into a name-calling match or irrational series of accusations and personal attacks, it's time to call the whole thing off and end the conversation. Ideally, when the conversation turns inflammatory, it's time to walk away 0 even if you're in the right. No minds are about to be changed and no ideas are about to be exchanged once the discussion gets heated.
The old adage, "never wrestle with a pig. You'll both get dirty, and the pig likes it," holds especially true here, and if the person you're talking to has more interest in getting a rise out of you, they'll behave irrationally and dismiss your evidence, studies, and any real proof you have to offer just to continue to get a reaction from you. If you know this is where these conversations will go before you even start them, it's best to avoid contentious topics with this person altogether, and save your sanity.
Step 3: Disengage Gracefully, and Leave the Door Open
When it does come time to disengage, try to do so in a way that both leaves good spirits intact and leaves the door open to resuming the discussion when the person has calmed down or has decided to stop being irrational. For example, saying things like

• "Look, we're not about to change each other's minds about this right now, but I wouldn't mind talking it over with you later when we can look over the facts," or
• "Well, there's no reason for both of us to be upset over something silly like this, why don't we talk about it later," or
• "Look, I don't want to discuss this anymore. Maybe we can talk about it another time when we've both calmed down,".

This is a fine line to walk—you don't have to leave the door open to future discussion, but since we're operating with the assumption that you're the informed party with accurate information to back up your position, there's no reason not to unless you know an informed discussion with this person just isn't possible. If you think they have something they can teach you, or a different perspective that can inform, reinforce, or even alter your own opinions, it can be valuable to converse calmly with someone who disagrees with you.
That said, you want to disengage in a way that leaves the door open, but says that you honestly want to stop talking about it right now. With the lines above, you do leave yourself open to an irritated person's teasing or accusation that you're the one who's irrational or the one who's upset, but you have to work past that and to the goal: ending the conversation right now and talking about something else. If you have to, go talk to someone else, leave the room to get a drink, do something to both end the conversation and alleviate the inevitable awkward silence that'll ensue afterward. If you've been honest, factual, and intelligent up to this point, there's no reason for you to worry about the atmosphere of the room after you've left, or the opinions of anyone who may have been listening to your discussion.
Step 4: Have a Friend or Family Member Talk With Them
If the situation is really bad, once you've exited the conversation, it may be worth asking a mutual friend—or if the irrational party is a family member, another relative—to talk to them and smooth things over. They may have to play devil's advocate and pretend to agree with them, but the point isn't to convince them that you're right, it's to convince them to simmer down and relax, and to lower the overall tension.
This step isn't essential, but it can go a long way towards maintaining or repairing a relationship with someone you care about if the person who gets irrational around you is someone you actually want to be close with, like an old friend or relative. Not every topic of debate has to be personal, and not every disagreement has to result in the end of a friendship. The sooner you both calm down and patch things up—even if it's through an intermediary—the sooner the whole thing will smooth over. Sometimes having a neutral party explain that the two of you should tackle the topic another time when you're both calmer (or there aren't as many people around) and capable of a more rational and reasoned conversation, the better.
Step 5: Let Cooler Heads Prevail: Win by Not Losing
As tempting as it may be to just smack the person upside the head and tell them they're an idiot, or shout "look how wrong you are!" until the person gives in and storms away, most contentious arguments in the real world don't work that way. Sadly, no matter what everyone on the internet tells you, a punch in the face or a slap on the head isn't going to change anyone's mind, or prove that you're right.
In the real world, the best way to win when someone gets irrational is to keep your calm, exit the conversation skillfully while leaving the option open to future discussion—because after all, you're the one with the evidence to back you up and the willingness to discuss it, and leave the other person blowing steam while you keep your cool. When you know for sure you're not getting anywhere, exit the conversation quickly, making clear that you could continue to prove your point, but see that there's no reason to continue.
Staying collected, sticking to your guns, and not allowing yourself to dip to the irrational person's level is the best way to deal with someone who won't listen to reason, or rejects your arguments out of hand. Staying cool and knowing when to fold are the important parts though. Your debate may not be as satisfying as you wish, but you'll at least be able to leave the tense situation and calm down knowing you were right, you behaved intelligently, stayed coherent, and left the door open. That's a win in anyone's book. Good luck!

How to Sound Like You Know What You’re Talking About (getting by)

These aren't instructions on how to win an argument or how to one up a troll on the internet. It's merely a guide to "getting by" in a conversation where you feel a little lost, or where you aren't as informed as the other participants. When in doubt, you're better off not saying anything, but these tips should get you through a conversation.
Project ConfidenceIf you act like you know what you're talking about, it's a lot more likely you'll be perceived as knowing what you're talking about. This means avoiding "blank words" such as "like", "um", etc. It's okay to pause and think when you have to, and if you accidentally say one of these blank words, don't freak out, but overall it's a good idea to try to strike them from your vocabulary. Talk slowly, calmly, and think about what you're going to say before you speak, and you'll already have a huge head start.
Know When to SpeakDon't jump at the chance to be the first to speak. Don't try and take over the conversation, either, especially if it's an argument. The last thing you want is to be called out on your knowledge gaps, and the more heated it gets, the more likely the loud people are the ones looking like idiots. Sit back, think about what you do know on the topic, and wait for a chance to jump in with that (don't just yell it out needlessly). Marketing weblog Tribal Seduction makes a good point: true experts don't just blab; they're very careful about what they say:
[Experts] know that people are paying attention to them, and that has two consequences. First of all, they know that their reputation is on the line every time they open their mouth—that everything they say will be subject to scrutiny. Secondly, they also know that people will put a lot of weight into what they say and probably act upon it, so they feel a strong sense of responsibility to provide good information.
Make sure you're not spouting off needless information just to be part of the discussion. Even if you aren't an expert, you'll sound a heck of a lot more like one if you're careful about what comes out of your mouth. If you can't form a coherent thought, then you're better off keeping quiet altogether
Emphasize What You KnowOver at weblog Nerd Fighters, they recommend you over exaggerate what you do know to make it seem more important, and to learn from the discussion whenever possible:
In certain cases, you can take what you've already said and apply personal opinion to it to add on to your statement. You can also make inferences from what the other(s) have already said. Agree or disagree with the person in cases where you will not need to present a reason why. Certain things that others say will sometimes fill in pieces of what you know, and that's probably your best chance at gaining knowledge to present.
If you can learn and synthesize information as the discussion is going on, you can jump in with points and act like you've had them forever, even though you formulated them mere seconds ago.
Don't Worry About Proving Others WrongYou're a lot less likely to come off as intelligent when you get in an argument and someone's poking holes in everything you say. If the group is arguing, you can take a side, but try to stress agreement with one side rather than disagreement with the other, if that makes sense. That way, you don't get stuck having to present evidence you don't have, but you're still taking part in the discussion and sharing your opinion (which, hopefully, you can present in an intelligent way using the other tips here).
This tip really comes into play when you can make a point that people may not agree with, or may not have thought about, but can easily understand. Weblog
Less Wrong explains this idea best:
At another point in the discussion, a man spoke of some benefit X of death, I don't recall exactly what. And I said: "You know, given human nature, if people got hit on the head by a baseball bat every week, pretty soon they would invent reasons why getting hit on the head with a baseball bat was a good thing. But if you took someone who wasn't being hit on the head with a baseball bat, and you asked them if they wanted it, they would say no. I think that if you took someone who was immortal, and asked them if they wanted to die for benefit X, they would say no."
    . . . [It was crucial] that my listeners could see immediately that my reply made sense. They might or might not have agreed with the thought, but it was not a complete non-sequitur unto them. . . If you want to sound deep, you can never say anything that is more than a single step of inferential distance away from your listener's current mental state. That's just the way it is.
The thinking applies to "appearing deep", but you can use it in a number of different types of conversations. The more common ground you can find—even in a heated argument—the more the other person is inclined to find you wise, intelligent, and respect you—and, often, stop the argument right there. And if you don't know what you're talking about, you probably don't have a strong opinion on the subject, so finding common ground with everyone should be pretty easy.
Steer the Discussion to Related TopicsIn the end, you'll probably exhaust your cache of knowledge pretty quickly, and won't have much else to say in the discussion. Unless it's a very heated argument, it's likely the topic itself isn't that important to those involved, so you can get out of it pretty quickly. Take part in what you can, then try to steer the discussion toward something related. If your friends are talking about a sport you don't know, move away to a sport you do know. If they're talking about politics of which you have no knowledge, try to find a jumping off point into something you do know. It isn't always the most effective, but if the discussion isn't too heated, it can get you out of a jam nicely.
Get In a Good Last WordAs things start to dial down in a given discussion, you have a good opportunity to be remembered and sound like you know what you're talking about. Weblog Productivity 501 explains the value of getting the last word:
If you have the final word and simply summarize the good points made by everyone else, people will remember your contribution more than the people who really came up with the idea. I'm not suggesting that you steal others ideas, but restating the best ideas (even when giving others credit) will make you look smarter.
Of course, if you've been silent for the rest of the conversation, this will look out of place and it'll probably be pretty obvious what you're trying to do. But if you've followed the above tips—sharing what you do know, learning and responding throughout the conversation, and finding that common ground, this can be a nice finishing move.
Final Thoughts: When Possible, Learn the Material
If you find yourself in this situation a lot, it's likely you get stuck in a discussion about the same topics—perhaps a talked-about political issue, or a a particular interest many of your friends share. If that's true, do some research on those subjects—even a bit of casual Wikipedia browsing—to boost your knowledge. This also allows you to do a lot of your thinking ahead of time, so you have well-formed opinions and know exactly how to present them.
Lots of people recommend accumulating random knowledge, and that can get you somewhere too, though only if done right. Memorizing the questions and answers in Trivial Pursuit is just going to make you seem like a weird cache of knowledge. Instead, get out of your comfort zone and read things you wouldn't otherwise read. I personally love the previously mentioned web service Send Me a Story—it'll automatically send random long-form nonfiction articles from magazines like the New Yorker, The Atlantic, and other outlets that you otherwise wouldn't have found for yourself.
I also love reading Reddit, and highly recommend you check out some of the other subreddits and checking out things like /r/AskScience, /r/ExplainLikeImFive, and other informative subreddits where people answer questions about complex subjects in plain English. You can also search through those forums for specific questions, in areas where even Wikipedia might be overly complicated.
Remember: in the end, it isn't about sounding like the smartest person in the room. You don't want to come off as arrogant; the goal is just to make it look like you're not the least informed person in the room. With a bit of practice, you should be able to handle most conversations with little difficulty.