If the Bruce Gilden videos can teach us anything, it is that you can get away with a lot, don't use a long lens, be quick on the draw (deliberate) and have a fast wit paired with a faster mouth. I will say that I don't think I'd be comfortable doing what he does, but his technique yields great results.
I think the single most important thing though is being quick with your shooting. You should be able to put the camera to your eye, take the picture and lower it within 2-3 seconds. Any longer and you're pushing your luck or you've probably already missed the shot. Oh, and you've gotta smile!