best way to keep the skins(specifically snare) out of the OH's is to learn the polar patterns of your mics very well, and position them so that the drums fall in the null point. cardioid/hyper-cardioid is obviously a good way to go, but if you have ribbon mics with front/back address, you can point the side of them at the snare and get great rejection. being in a large room with as little reflections as possible will also help.
