Don't think of it as jumping on the serve. Think of it more as adding some leg drive to your serve mechanics. With sufficient leg drive you will naturally come off the ground.
If your toss is relatively low (just high enough for full extension of the racket arm), you want to bend your knees as you are lifting your tossing arm or as you are releasing the ball. If you bend your knees after the release, you should toss somewhat higher than your contact point. Let the tossing arm continue upward (to vertical) after the ball release. Leave it up there until your racket head drop (the backscratch phase).
This should give you a better shoulder tilt for the trophy phase. The outstretched hand will also give you a better reference for the position of the tossed ball, which should help you time the upward swing to the ball and help you to execute the proper swing path.
The bent knees should extend as the racket head drops from the trophy position down to the scratch position. Your legs will be fully extended as you are start your upward swing -- your legs drive upward first and then your racket arm drives the racket upward immediately afterward.
Some aspects of the Roddick motion can be copied but his motion is a bit too difficult and idiosyncratic to use as a model. You might adopt his abbreviated takeback but his stance and some other aspects of his motion might result in some issues.
Federer would probably be a better model for you to study and adapt. Also take a look at Sampras. However the timing of Pete's serve might be more problematic.
Last edited by SystemicAnomaly : 05-14-2012 at 11:32 PM.