To defend the break, it's all about position. Instead of crashing the boards, have your defense get back on D. Do this by lowering crash board to zero. This limits everyone on your team from trying to go for the offensive board and just focus on getting back down the court to defend the break.
Also on the break, you want to stay neutral if possible between the ball handler and the recipient so that you are in position to defend against any one of them taking the shot. Also, don't be afraid to foul. Fouling is not always a bad thing. It's better to foul the man and make him miss than to give him the easy basket. Make him earn the two at the line. In some situations, it's better to just give them the shot rather than foul. You have to use your basketball IQ to discern when to do what in the given situation.