A rather broad guitar strap with a "roughness" or "nap" on the inside - the spot that touchs your shoulder - will help. Once you get the angle of the neck & body where you want it, the "nap" will help it stay there without sliding around too much.

I've been struggling with this problem recently while fingerpicking a brass bodied, resophonic guitar that is real bottom heavy and wants to droop away to my left. The broader, "grabbier" strap has helped somewhat.