Yeah, practice the armdrag.
Have an opponent on their knees and practice popping your foot out from the butterfly guard, using your cross hand to pull their arm to the mat, reaching around to their back and hooking under their opposite side arm (this hook is amazingly helpful in securing the back).
Practicing this drag is everything.
I also practice the armdrag from standing, but it's a little different than off of your back (or, more accurately, off of your butt, which is where you should be in the butterfly guard). Still, it'll help you develop the tricepts and back muscles required for powerful armdrags.
would you happen to have any pictures of this drill at all available? thanks for your time mate.