Make sure you are pushing your arms open as you kick over. I see many people closing their shoulders as they kick over and wonder why they are falling on their face. All of the drills BB listed are great as well. If you are trying to start from a stand, make sure you think of the bridge as something that happens from the top down. I like to make sure the kids do the following:
Reach up (this actually allows the shoulders to open more)
Look for where your hands are going.
This method saves the lower back from undue stress. Many people bridge quite easily because of lower back flexibility but most of them complain later of lower back pain. I have some girls with the stiffest shoulders and lower backs in the world that can still do back walkovers since they are using the proper technique and they are strong enough to push over even if their angles are slightly off.