I concur with the points above. Racquet speed/acceleration is key. Loose grip, flick of the wrist, shoulder rotation, bending the knees and propelling up to the ball are also keys to generating power.
There was a clip around that illustrates Roddick's serve that might be worth taking a peek at. (I'll try to remember where I saw it)
Also.........the USPTA has a video clip on the flat serve, using Sampras, you might find of interest: