TRA made a guide which should have been turned into a sticky. Too tired to search for it though. If you've got your technique right but can't hold the bike on the rear wheel, I suggest you just use less speed, don't lean forwards so much when you're doing the tap and try to pull the bike up and ahead of you, not up and under. Another thing might be not to hit the front wheel too low. If the walls are under say 40", try hitting them hard on the edge.