Plenty of good advice above.
Video to teach deeper racquet drop (notice how far to the right side of the body the racquet drop is:
McCraw Serve Fundamentals http://www.youtube.com/watch?v=WuIgT...eature=related
Pronation practice to incorporate that deeper racquet drop into a proper arm motion:
McCraw explaining a serve pronation exercise http://www.youtube.com/watch?v=iONY6fcqZGg
Robin Soderling shows the deep knee bend "archer's bow" configuration (with your front hip well over the baseline) you will need to use your whole body to power your serve:
Robin Söderling serve slowmotion http://www.youtube.com/watch?v=a56pvP1i6x8
The above clip of Soderling is a very short version of the information in this video
Nick Bollettieri-Sonic Serve.wmv http://www.youtube.com/watch?v=ajoZ0f7hw-A
One thing that confuses many is WHEN to "bend the knees" and "go into an archer's bow".
Notice as Pete releases the ball in pic 1 he has only a small knee bend (but do NOT straighten your legs like too many do as you release the ball up - it becomes too hard to straighten your legs as the toss goes up, then have to get even this slight bend in the knees as you release the ball).
It is while his tossing arm is pointed straight up that he "bends his knees" (you will feel it more as lowering your whole body weight, as in a squat.)
Lowering your body weight will let you stay in balance as this is like a controlled fall - almost like sitting down but with your tossing arm straight up, as seen in pics 2-8.
Notice that it is fairly late, in pics 8,9 that Pete forms the archer's bow shape.
As a server, what you are trying to do is get your hitting shoulder as close to the ground as possible - but as you do it YOU HAVE TO LET YOUR FRONT HIP SWING OUT OVER THE BASELINE to maintain your balance.
If you can get into this position in your trophy pose with your knees very bent and your body in a bow shape ...
... then all you have to do is push off from your legs as hard as possible and reverse your bow shape UP and at the ball to get your body into the serve.
So I urge you to practice this motion at home without a racquet to get comfortable with bending your knees and getting into a bow shape ...
so you can explode out of this position to power your serve.
(If you wait until you get to the tennis court to try and add this into your serve, you will feel way to rushed, and it will take much longer to get it.)