Power Serving

The First Source of Power is a Loose Grip.

Where many go wrong is by holding the racquet far too tightly at the beginning of the swing. It is essential to stay loose on the serve. To get the idea, hold on to the butt-end of the racquet with two fingers and your thumb, then try your serve.

The Second Source of Power is Proper Timing. A sequence of events takes place in the serve that must be properly coordinated.
First event is the back-swing and weight transfer. From a ready position with your hands at the level of your waist. Make this part of the serve very deliberate and precise. Start slowly so you can finish fast.

The Third Source of Power is a Complete Elbow Bend. 
The racquet drops down behind the back as it coils for the upward spring. (To see if you're getting enough elbow flex, try serving a few, touching the small of your back before swinging up.)
The shoulder turn follows the elbow bend. The shoulders roll through to facilitate the upward movement of the elbow. (To check if you are coordinating this properly, hold your follow through position to see if your right shoulder (for right hander’s) is lower than the left.)

The Final Source of Power is the Wrist Snap. 
At the point of contact the racquet head is thrust upward and outward. (Hold the racquet in the middle and see if the butt rotates to the outside of your forearm.)

Keep your power serve under control. At the moment of impact try to keep your balance. If your body is out of control, your serve is out of control. Keep your head steady with your eyes and chin up until after the ball leaves the racquet. Remember, you're hitting up, not down.

Now get out and show Roddick how it should be done!!