Dunking difficulty? Again?

This is my second question on dunking. Any tips? I'm 13, 6'2, weigh 170. This is where I am at dunking. Usually when I get up there, I loose hold of the ball and it just goes forward into the backboard. a href="http://i54.tinypic.com/ftlkp3.jpg" rel="nofollow"http://i54.tinypic.com/ftlkp3.jpg/a