I've told this story here before, but it's worth repeating. A friend and I once debated which pocket was best for the wallet. Front or back. I said front, he said back. His reasoning was that with his shirt untucked, no one could get his wallet out.
I told him not to bet on it, that I'd have his wallet by the end of the day. I had his wallet within 10 minutes of the conversation. Lunch time came and he freaked out and said "I think I lost my wallet." I smiled and handed it to him. He put it in his front pocket.
Besides being safer, it's just a lot more comfortable. I keep spare keys in my wallet, I don't really want to sit on those.
I carry a fairly large wallet (Spec Ops THE wallet... the little one) and an Alox scaled SAK in the left pocket and never feel like I have too much. The right pocket is usually empty other than my cell phone in the watch pocket (if there is a watch pocket) and a clipped folder and some change down in the bottom.
I wear cargo pants at work and keep my glasses case on one side and a small notebook in the other. There's never anything in my back pocket. Don't know why. I just never put anything back there... don't want to disrupt the lines of my sexy arse.