It's part material and part weight. Using buoyant material like bucktail causes the back to ride up, as does counterweight. Not sure if you recall the "jelly belly" pattern I came up with years ago but it uses a chrome-lined soft plastic wrap which combined with buoyant material requires no additional weighting to ride up.
Experimentation is the best teacher.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum