Sorry to hear that!
I've been hisses at, bitten, constricted, even pee'd upon, but I have yet (knock on wood) to be pooped on by a snake. My green-eyed gecko gave me the honor oncec or twice, but little baby gecko poo is nothing in comarrison to adult corn snake poo. I'm sure it'll happen eventually, but I'm glad it hasn't yet!
Anyway, about all the advice I can offer is here for ya! Foremost - do not put the snake away after it poo's on you; you don't want to "train" it to think that pooing on the handler is the way to get it's privacy back. Just be ready and have some paper towels on you lap next time you handle it so it's not a major mess. Next - try to keep the stress to a minimum when you handle it... if possible, let it come out of it's cage on it's own, for example (every time a snake has pee'd on me it's because it was scared, and tried it as a defensive measure.)
Maybe another thing would be to NOT handle it at all for 2-4 weeks. Let it establish a pattern of defacating in it's cage first.
Good Luck!
^Curtis