There are lots of different exercises to help with this, weight training for stronger legs and back, special shoes, with the heel lower than the toes, to encourage the use of the right muscles, but the best way is to learn and practice proper technique. That means using your whole body. Find a wall and take a quick jump and touch the wall. Then mark that spot. Proper technique involves 3 steps, feet together, bend the knees, bend at the waist, bring your bent elbows back behind your back. Then everything comes uncoiled at once. Bring your elbows forward and up, straighten up your back, push with your hips, thighs, knees and finally toes. Go back to the wall and see how much higher this technique gets you. Used in conjunction with a weight training program to strengthen your legs and back, this technique will get you above the net.

I know a lot of youth basketball players who use a bike and jump rope.

Proper technique is very important. Without it all your strengthening will go to waste. You have to set your feet, and then bend at the knees and hips. Keep your back straight and throw your arms back behind you. Then throw your arms up and push with everything, your back, hips, thighs, and calves and toes. Picture being worth a thousand words - watch this: http://www.youtube.com/watch?v=NhvISlxCqVs