Express your anger with words, and then walk away from the situation. I like to play video games to take out some of my frustration. Exercising is also one of the best things you can do. Play an instrument, listen to music, whatever relaxes you :)
Why wouldn't you want to fight? You should scream into your pillow if you really don't want to fight, or have your parents do what my parents did a few years ago, have them by a punching bag for you, or join a boxing club.
Become arrogant. Chances are you are probably better (in some ways) than the person you want to fight. Realize that you don't want to lower yourself to them. You don't have to say it to them, but you can let your friends know later that's the reason you chose not to fight.
Well, I usually go and punch a brick or a wooden board a few times, or spar with someone for a bit, and then I feel better, be careful if you decide to do any of these though, I've broken all my knuckles and fingers at least once doing this, some of them more than that, so proceed with caution.
Don't do dumb things like preppyboy Your wasting your energy becoming emotionally involved with situations that require fighting.. Whatever drives this stress related reaction... you need to ask yourself if its important enough to keep in your life