The top servers in pro tennis all have one thing in common: they flex their leg muscles just before they stretch up to hit the ball.
Most club players, on the other hand, are too stiff and upright when they serve. As a result, their serves lack power since the arm and shoulder are forced to do all of the work.
The next time you practice your serve, flex your legs as you drop the racquet head behind your back. This flexing action will allow you to use your leg muscles to spring upward and stretch out fully as you accelerate the racquet head through contact with the ball.
So for a more fluid and powerful serve, don't forget to flex your leg muscles!