shave your ass.
No, really, I'm dead serious. Hair getting tugged by clothing is a major cause of sores, because the agitated hair is treated as a foreign substance by your body.
I would advise against shaving. Shaving bumps and ingrown hair could make things worse. If you must maintain because the other suggestions fail then pluck the offending hair. I know, not fun but better than sores.
Edit: These days waxing is also an option...