Yeah throwing my head back is one of my main problems...
Watch the bar throughout the entire skill.
Try to do 2 in a row instead of just one. Sometimes folks fall because they are trying to stop.
I'm guessing you're currently trying to get your momentum by pushing your feet around the bar. I don't know about you, but my feet don't weigh enough to move my shoulders around the bar.
Back hipcircles 101:
1. start on top of the bar, leaning slightly forward.
2. Cast up. (far too many kids cast away from the bar instead of above it.) If you feel like you're going to fall on your face in your cast, it's most likely where we need it.
3. As your hips come back to the bar, fall back with your shoulders while keep the rest of your body in a tight hollow.
4. motor cycle hands. In the words of the band .38 Special, "Hold on loosely but don't let go"
I know it's a bit of an over simplification but I hope it helps.