Do a lot of weightlifting... like squats of jump shrugs with dumbbells. It helps ALOT.

We do an approach jump over a teammate who is on her hands and knees.
This makes the jumper get up into the air. Of course you will need to be next to the player on the ground so she does not get hurt. We have added several inches to each players vertical jumps over a short period of time.
Hope this helps. We even did this activity at the Far Western Tournament in Reno, NV

use air alert 3