Plyometrics are excelent for increasing vertical jumps. So are hevy squats. One technique I used while aiding my friend who is a collegiate volleyball player, was to train legs unilaterally. We used techniques like split squats. Also do not overlook the importance of sprinting, these also help in developing those type 2 fibers.
I have a friend who's a pro beach volley ball player and here's one of things she does:
After squats ( or split squats, prob. doesn't matter), she jumps onto a bench--started with those stacking benches for step aerobics and then worked up to a weight bench. This could be a little tricky in a really crowded gym, but when's it's empty, why not?
Plyos, such as body-weight jump squat (squat down and jump up as high as you can, repeat until your quads can't take any more, rest, repeat), box jumps, etc. Jumping rope also helps a lot.