More than drills, I've found that you need to have VERY good shoulder and upper back flexibility, or you get that bent kneed, legs apart, on their toes, shoulders moving forward, bent arm sort of thing. All my girls that can do that skill well are able to put their hands at the base of a wall in a bridge and push up in perfect form and have their chests flat against the wall. It takes time and work to get the amount of flexibility, but when you do, you will find it pays off in many other skills as well. Keep at it, and good luck.