First you have to be very calm and just be nice and avoid comfrontation. If they keep going let them get close and in your face, stay calm the whole time. They ususally don't throw the first punch if you just stand there. Wait, once they are really worked up,(without making any telltale signs that anything is going to happen) throat punch the first guy. Then give him a very hard punch to the solar plexes. If there are other people trying to start crap, just go hard and fast towards them.
But most people will be pretty off gard if you do it right. Some people back off after their friends get messed up. Without a "leader" most people are pretty chicken, since usually the "tough guy" will be the one trying to start it. If not you lay into them. The basic thing is that most people don't have any training, so they are easy to take down, and most people don't expect a calm person to react fast.