Another usefull piece of advice, I'd add, is don't carry around your credit cards unless you absolutely know you're going to need them! If I'm just running out to class or to do a few errands, there's no need to keep 2-3 cards (or more) in my wallet. I try not to carry them at all these days, in fact, unless I'm travelling and/or know I need one for a purchase. The rest of the time I only carry my ATM card.

I had my wallet stolen about 10 years ago with 3 credit cards, an ATM card, Social Security card and all sorts of other stuff in it, none of which I'd needed at the time. Lesson learned the hard way!