"Let's assume that what is desired in your serve is more power. The next step is to picture your server with more power. One way to do this might be to watch the motion of someone who gets a lot of power in his serve. Don't over analyze; simply absorb what you see and try to feel what he feels. Listen to the sound of the ball after it hits the racket and watch the results. Then take some time to imagine yourself hitting the ball with power, using the stroke which is in as much visual and tactile detail as you can. Hear the sound at impact and see the ball speed toward the service court."
From "The Inner Game of Tennis" by W. Timothy Gallwey