Learn the hs backroll with both hands on the handle.
It's hard to say, but it sounds like sean has the right idea. The body position you have when you leave the wake always seems to be the way you land.
When I learned the back flip on my trick ski, we were always told to edge away from the boat... Its really hard to explain, but picture yourself cutting into the wake toe side for a toe side back roll. Instead of letting the boat untwist you and open you to the boat as you hit the wake, you want to almost be twisting away more with your back shoulder as you go off the top of the wake.
If that makes any sense, it's the same for the HS roll. When you are hanging outside the wake the board is facing frontwards (parallel to the wake). The closer to the wake you get, the board should become closer and closer to perpendicular with the wake, and you almost try to get it PAST perpendicular to the wake as you go off the lip of the wake. This has the effect of bringing the board under you more when you take off. (and hopefully when you land!)
I don't know if this makes any sense, but I hope it might help.