The next progression for increasing the power of your serve is the squat jumps. Since power and explosion are paramount in service power, the squat jump is a perfect tool to use.
Progression 2: Squat Jumps
How to do it:
1) Stand with your feet shoulder width apart
2) Slowly bend your knees and lower your hips as you were performing a squat
3) When your thighs reach parallel, drive your feet into the ground & thrust your feet forward, while exploding up into the air
4) Land with your feet shoulder width apart
5) With a fluid motion repeat for 10-20 reps
Beginners: For beginners, it may be more difficult to smoothly transition from one squat jump to the next without pausing. If you are having trouble, between each rep, pause for a second to gather yourself and repeat.
Intermediate: For people who have good stability and balance, the goal here is to perform one rep to the next in a fluid motion without pause. To add intensity, each progressive set work to decrease the time your feet touch the ground on each rep.
Advanced: To really increase the intensity and activate fast twitch muscle fibers used in the serve, add weight.