Jumping technique just comes with practice. Experiment with it a little.
Check out: www.howtojumphighernow.com
for some good (and free) information on improving jumping ability.
Oh...another thing to mention, and I'll mention it because you don't see it on the jumping web sites. It may sound silly but it works: visualization. Spend some time every day visualizing yourself jumping higher. If you're trying to dunk, mentally "feel" exactly what it will be like to dunk. See yourself doing it in your head over and over again successfully. Make it a mental reality. Funny as it sounds it really does help.