I keep two wallets - one in the back pocket with a little money and some stuff I wouldn't mind losing, and a separate one in the front pocket which I take great care to be sure it folds flat (so no bulge due to wallet).

This way, if some geek runs up to me with a knife demanding my money (urban survival...) I have something to hand him/her. Usually they just want to get away in a hurry, so they'll take off with that. And if he/she makes me start turning my pockets out you can bet I'll keep him busy handing him useless (from his perspective) stuff <img src="images/graemlins/smile.gif" alt="" />