The trick is to not care. (among other things that people have mentioned above)
For example, one of my friends in high school has his pants pulled down infront of like half the school… but instead of running away in an embarrassed fit, he left them down for a sec and turned around with his hands up as if to say 'My pants are down… whaddya think of that?"
People watching may have thought alot of things, but I bet what they didn’t think was ‘that guy sure isn’t sure with himself’. If you’re not attracted to yourself… no one else will be, unless your some lame emo who somehow manages
I found that rapidly and repeatedly head butting anyone who did such things was an even more rapid way to boost self-esteem. Pulling pants down in horseplay, headbutting can leave visible scars…