There was a recent thread which discussed this subject that had some good advice:
http://www.wakeworld.com/MB/Discus/messages/65919/714919.html The key to grabbing any invert is to look at your board when you reach for the grab. Also, really concentrate on the grab and don't worry too much about the rotation of the backroll. When I was first learning to grab my backroll, I was too worried about getting my body/board all the way around and sticking the landing, so I never ended up connecting my hand with the board. I would reach for the board really quick and then think "Oh no, I'm gonna crash!" If you just focus on planting your hand on the board, your rotation will take care of itself naturally.