Having enough wrist strength is a crucial part of an injury-free handstand training. Furthermore, not everyone has enough flexibility to keep their hand back 90 degrees for an extended period of time.
In this video by Chris Silcox you can learn several techniques to improve your wrist strength, flexibility and general upper body strength needed for various hand-balancing moves.
If you’re looking to truly make your entire body indestructible, check out this guide to improve mobility, flexibility and strength.