Haven't seen a thread like this here yet.
I'm always interested in things of this nature because what is common sense to one might have never occurred to another. I'll start with a few:
1) When setting "home" in a GPS, set it a few blocks away. This way if your GPS is lost or stolen, the thief won't have your home address
2) Always fill in the address line last in an email. This will insure that you don't send an incomplete email, or one laden with emotions that don't belong.
3) For the ladies : Use a flat iron to iron out creased collars or hem lines. Chances are, you're going to use it for hair anyway. (For the ladies because most dudes I know don't have flat irons)