Lawnjart mentions in how to fight and kick ass that a good way out a fight is to suggest that you'll viciously attack the person after they start, but that this is unlikely to work. I present an illustration of this technique's effectiveness:

It's 2:30 am, this morning (that's about 2 and a half hours ago now). We're walking out of a frat party. 4 guys in a Jeep Cherokee pull up in the praking lot, real fast. One of the guys in the back rolls down his window, yells to my friend, "Hey, Come here. Hey, you friends with Carson Daily?"

But you know, it was the way they said it. They're obviously drunk, looking to pick a fight.

My friend walks up to the car's window, puts his hands up and motions to himslef and asks, "You want me to kick your ass?"

The guy in the back, previously looking to pick a fight, just says "Drive," and they roll the window up and pull out fast, without another word.

I have to make a few disclaimers though. First, we were pretty drunk, so my friend had a lot of confidence. Second, He was already pissed off, so a fight was just what he was looking for. It all lends up to this reckless confidence that's pretty hard to fake. If you want to get out of a fight, and you think the other person's at all reasonable (you have to be fairly certain of this), just make sure you look like you're ready to go out of your mind on them. Nobody fucks with crazy people. Of course, if you even care about whether the other person's rational, you probably can't pull off this insanity. Irony's a bitch like that.