Body rotation and racket movement are dependent on each other.  Proper body movement in a figure 8 motion creates a more powerful swing.  Practice without hitting a ball to acquire consistency with your movement.
EXERCISES
1. Fix the Hitch - concentrate on feeling hip rotation in order to drag your arm forward.
2. Transition to ServeMaster - exaggerate driving the back leg early for hip rotation.
3. Feel it with the Racket - Keeping your lower body moving through the serve, keeps your racket moving.
 "Understand the Kinetic Chain."