Get a drink. Count to 10 (I'm serious).
Release your anger on something of little to no value (you know, like, bending a guitar pick a bit).
Evaluate why you lost. Analyze every little mistake you made. Watch the replay, watch the demo, think about what your opponent was thinking, how you can be less predictable, always have the jump on him...
Play with a clear mind, free of anger, free of fear, free of emotion. Play like a machine. Toy with your opponent the next game.
And hey, if it's a team game, "FUKKEN TEEM MATE LOL!"
One can only be expected to do so much in a certain situation.
Granted, in Quake, a 1v5 is certainly possible if you isolate your opponents and play cleverly.
In WoW, say, class > gear > "skill", and a level 75 in a 70-79 bracket WSG against 5 79s in an open area can only do so much, you know?
Hold yourself to high standards, look for improvement, but don't set the bar to unreachable goals.
And we all have bad days, and good days.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sometimes, you're the windshield. Sometimes you're the bug.
Sometimes you're the bat, sometimes you're the ball.
It just works like that. Put it down, do something else, come back in a bit. If you have to keep playing, like for a clan tournament or scrimmage or something, then you just need to train your mind.
People don't believe me when I say most gaming that involves competition takes place 80% in your mind, and 20% in the game (10% aim, 10% movement, etc.)
Learn what to do in certain situations and it all falls into place.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Edit: If you're like me, and you have issues with remaining calm sometimes, relaxing music really helps. Even if you're playing a game where people talk about listening to speed metal while playing, just remember...Calm, Cool, Collected, and Focused.
Music from the 'Braid' soundtrack usually helps me out well.
http://www.youtube.com/v/5kflCLshkOg