Alternating swings with a heavy racquet head and regular swings will help players learn to accelerate the racquet head better. Here is a drill using this concept.

Load and Repetitions:
You need to be careful to use enough weight to make a difference but not enough weight that could injure the player. Also the loaded swings have to be executed quite fast. Ideally you should alternate 4 to 6 shadow swings with 4 to 6 normal serves. The player should try to hit the normal serves as fast as possible and repeat the process 4 to 6 times.