From my experiences:
1. Cut your toenails as short as possible. Long toenails make for less room in your boot so your foot has less room to slide before slamming into the end of your boot and so that if your toe is forced up, the toenail won't be forced down at the end causing toe bang.
2. Make sure your boots fit - too big and your foot will slide forward causing toe bang. Too small and your toes will be too cramped causing toe bang.
3. Don't land back seat - this will cause your foot to be pushed forward upon impact with the ground. Try to land as if you weren't wearing skis and you just jumped with shoes on.