I got boxer fracture for the 2nd time. What can I do to prevent another fracture?

Bad Punch Technique? Assuming osteoporosis is not a factor, most boxer's fractures come from poor fist fighting technique. With a boxing glove, the most effective punch is the roundhouse, hooking type that get ist power from centrifugal force. That same punch thrown without a padded glove results in the angled fracture of the outer hand bones behind the knuckles of the 4/5th finger. Use a linear straight punch!
Boxers fracture. If you're a real boxer and getting these, work on your technique. If you're getting these because you're angry etc.. , maybe consider some form of anger management, stress relief. This all assumes that you have normal bones i.E no cyst or enchondroma present. To make sure of that, you should have a consult with a hand doctor.