If you're about to have sex with a girl or she's about to go down on you, before things get rolling excuse yourself to the bathroom and clean out your butt crack. First wet a piece of TP and wipe, then finish it up with a dry piece of TP. You need to do this because when things get hot and heavy you'll start to sweat and that'll make any leavings in there stink and no girl likes that. Make sure you do this even if you think you do a good job of wiping because not matter how good a job you do you'll be surprised how dirty it gets down there.
Sorry if this is too graphic for anyone.