|Taking it out on an NPC could be a good way to release some anger|
Don't take every fight
If you, like me, like to talk too much, and especially argue way too much, you might think every statement made by a party member is an invitation to discuss. Rule #1 - they very rarely are. Unless you happen to agree with the person, let it go! Unless it's so ridiculous a statement that you just can't drop it, like something racist or the like, you don't have to start a debate right here and now. The random pug is definitely not the time and place to sort out the rights and wrongs about many things, and if the group is moving forward everything is working ok enough! In either way, you won't get the other guy to change his mind during this short period of a time anyway, so you might as well not get started at all.
An example: Yesterday I did a random run into SM Cath. The first thing the mage in the group says is "Oh great! Finally an aoe-tank", when she sees that we have a pala-tank. I was just about to open up Pandora's Box on her (it was a female char) on the fact that all tanks have good enough aoe-threat on low levels, that I personally actually think paladins have among the least good aoe-threat since it's so stationary bound and alot of other things. But I didn't. What would it have brought me? The mage wouldn't have agreed with me for sure, she clearly thought paladins were the best aoe-tanks, and why not let her think that? It was probably based on her experiences, and who am I to say those are wrong? Instead we started to casually talk about how hard it is to get a job nowadays, something everyone can agree on (unfortunately). So, pick your fights carefully when in a random pug. They're very rarely worth the trouble.
Be silent, or say something nice
Like my mom used to say - if you can't say something nice, don't say anything at all. Like I mentioned in another post, there is a time and place for when you should and could tell people that they're not doing it right. Unless your group is having serious trouble getting the instance done, or the guy in question specifically asks for it, keep your good advice to yourself. Unfortunately very few people think you're doing them a favor by offering advice, they're just seeing you telling them they're bad. If you really feel it's needed, at least follow my steps in the abovementioned post! If you're going to comment on another player, try to find the good things. I usually always thank people when they do things people don't normally do - even if you might expect them to do that. Like say silence that nasty caster standing over there. Offhealing when the regular healer seems to have gone afk. Conjuring water. Not needing on those pants because they already had fairly good ones (ok, that's not going to happen). It's just as outside of WoW, you do want a thanks for doing something, even when you're supposed to do it! We don't have to go as far as "thanks for tanking" - that's just going to be interpreted as being sarcastic or smug. But giving someone a compliment is rarely wasted time. Saying someone sucks won't get you very much though. Remember the goal of you being here is getting the instance done after all - not getting the satisfaction of telling people off.
Assume people don't know, rather than don't care
When someone does something that really ticks you off, always presume they just don't know how to do it the right way. This too is something I've mentioned before. When someone does constant ninja-pulling, needs on everything, uses the wrong skills or just overall makes your life miserable, assume they haven't learned the right way to do it rather than that they're doing it to specifically to make you angry. Unless they're actually telling you so. One of my favorites is when someone in the group goes "Oh look how this guy is geared/gemmed/specced, let's kick him!". Like the gear/gem/spec was a personal insult. I agree in some cases it actually could be, but we've all been noobs sometimes, and we never suck at something to annoy someone else. If the necessity arises to tell someone they need to go about doing things differently if they ever intend to be able to play with other people, see abovementioned post for some pointers on how to proceed.
Don't take things personal, even when they're personal
This is similar to the previous pointer, but alot harder to do. This time I don't just mean that you shouldn't take bad playing personal, I mean you should try to not take personal remarks as personal. It's near to impossible, but probably the most important part in trying to keep things cool. When someone tells you you're doing it wrong don't become all defensive at once. I personally suck at not becoming defensive since I really suck at taking criticism. There are two ways people can tell you off (or give advice) and two (proper) ways for you to react.
Either they can just say "tank/healer/dps you suck". Unfortunately this isn't very constructive, and even more often not even very correct. The main reason the guy is telling you this probably is because you're not doing it the way he wants you to, simply. The best thing is to ignore this kind of comment. Sure you hate the guy who said it, but imagine him being a sad and lonely person who needs to tell people they suck so he won't feel so bad for himself and put him on /ignore and just continue on your way. If he really hates what you're doing he'll hopefully leave group. If he acts an idiot in other ways aka ruining the run, you might want to consider leaving the group. But don't take it personally.
Or they offer some honest advice. The other day when doing Saurfang on my hunter a guy in the raid whispered me some pointers on how to best handle the Beasts, for example. Instead of taking it as an insult to my intelligence, I was happy he wanted to share his experience on the matter to spare me the trial and error period.
To this you can either notice that you're actually learning something new and thank for the advice, or if you don't agree with the advice simply say so. "Thank you, but I don't agree :)". Or "Thank you, but I prefer it this way :)". Or "Thank you, but this works for now, I might change it later :)". Well you get the idea.
Realize the world doesn't center around you
As a tank I often do think the world centers around me. I still believe with the current shortage on tanks, and the way tanking works that they're doing most of the job, especially in lowbie instances where tanks often also provide half the groups damage. But for your own blood pressures sake, it's best to not try to always have it your way, or the high way. I've tried to play this way for years, and it just gets me angry. I used to be one of those tanks who just let people die if they pulled when I had said I was going to eat/check my addon/wait for the healer to get mana. If someone does that too many times I still might. Or maybe I want to take a certain path in the dungeon or skip a certain boss, and no one else agrees - well it is a group effort (even if you're 80% of the effort). Come to terms with the fact that you can't make people do exactly what you want, so you might as well not hope for it. And most importantly, not be angry about it. I could of course tell them to please do it the way I like - don't ninja pull, wait for me to get mana (when healing or dpsing) and the like. It works more often than you'd think, so it's definitely worth a try. But most of the time I just think "ok this is how it's going to be". I'm not saying it's easy, because it's not but try to suck it up and see it as a challenge instead.
So everytime you manage to get through the instance without too much emotional hurricanes, give yourself a gold star!