You know those funny tight pants Lance Armstrong wears? Those aren't just to show off his amazing legs... No really!
They have padding in the crotch (and usually about halfway up the butt) that helps A TON. They aren't cheap, though, so unless you're planning on doing multi-hour sessions you might want to try something cheaper. Maybe get a big fat menstrual pad and tape them to your butt? Might look funny, but if you wear loose pants no one will notice (hopefully).