Tips
This small progression chain can be very useful to get rid of backflipping fear. But note that you jump backwards in a backhandspring and in a backtuck you jump up.
Lean back before you initiate the jump. You don't want to jump too high or else a lot of momentum will be lost and you'll have trouble coming back onto your feet again.
Look back as soon as you can, so you can spot the floor as early as possible.
Some back and shoulder flexibility will definitely help, as you can reach back further before you have to jump off. The bridge position can also be a useful exercise.