I personally like to "surprise myself" and carry the extra mag in a new place everyday. Mondays I prefer it in my left front pocket, tuesdays I really feel better if its in the right front pocket. However on Thursdays I like to cram it in my right rear pocket between my wallet so it messes up my back when I sit down. And on the sabboth I have a lanyard that I hang the extra mag around my neck with.
Seriously though? Left front pocket.