My boys are in full coat and I trim very little on their backsides. To prevent problems, I try to make sure they are combed. Tangles are usually what cause problems. The cleaner and better combed the hair, the more things to seem to work out okay. Max very rarely has problems, but Teddy sometimes needs help. He always comes to tell me.
For the belly band issue, make sure long hair stays out of the target area when you put the band on. Change frequently and give a belly wash daily (wet washcloth or wet wipe).
We usually give full baths once a week, and mid-week a half body wash.