Something else to look for are ruts and berms...they are your friends!
In slippery conditions, a rut or a berm will hold you much better than trying to slide around in the smooth part of a turn. Many people go way inside counting on the fact that they will drift and slide to the outside. The problem with that approach is that when you do get to the berm, you're more likely to slide over it. If you're pretty sure you're going to slide out to the berm anyway, start out there and use it all the way around the turn.
Another thing is to ride a gear high, your big 4 stroke will be able to pull 3rd around the turns and careful throttle control will help keep the back end from breaking loose.
As you mentioned, if it's muddy and the rear end is slipping out, you're either too far forward or getting on the gas too hard (or both). If the front end is sliding out, you're not far enough forward. Move around until you find that, when going too fast, both ends are drifting equally in the turns.