Pronation and the ability to hit "up" to the ball is one of the most important steps of developing great serve technique.  The racket head will drag behind the hand creating a 180 degree arc if done properly.  Pronation is natural, but you must practice your timing for best results.

EXERCISES
1.  Using a full service motion, relax the hand and entire arm. 
2. Let the wrist flop over. Use ServeMaster to make it easier to FEEL as the weighted end pulls your wrist /arm over.
3.  Practice with the racket to improve how the racquet and hand movements together create racket head speed.
4. With an abbreviated swing, learn to isolate the most important elements that you need to improve in your serve.