wipe the stuff off with kleenex (toilet paper sticks to you like crazy; its friggin annoying) as soon as you come, then wash yourself off (always keep SOMETHING within close reach to wipe with)
Oh, remember, just because you came doesn't mean you've ejaculated all of your semen. Quite a bit more might slowly come out over the next half hour, so (this is weird talkin about this) try to squeeze out any that might still "be in there" after you come. I'm pretty lucky; I'm able to have multiple orgasms (which most guys CAN'T do) so any semen that might be left comes out within the second (or third, if I'm feeling REALLY horny
) time I come.
So I hope this helps (I'm gonna start a multiple orgasm thread if anyone has questions about how you can do this, cuz some guys can like me)