I think serving is very, very much like pitching a baseball. See Nick B's sonic serve video: http://www.youtube.com/watch?v=ajoZ0f7hw-A
This video is a better example too: http://www.youtube.com/watch?v=6EFWB18kPWY
I had a left partner for a number of years who pitched collegiately D1 and he developed a good serve.
Next time you practice serving, put down your racket and put a tennis ball in your racket hand. Go thru your service motion and throw the tennis ball smoothly up through the contact point. The ball should go high and forward. Repeat 5 or 6 times. Then shadow stroke serve with racket in hand and think of throwing the racket head up through contact.
Then practice serving and continue to think throw the racket head up and into contact.
Get your racket hand outside of the ball at contact and think throw up and through the ball on an 8 to 2 o'clock racket path and you should have the making of a good top/slice serve.
Disclaimner: I am not a coach so if I screw your game up it's your own fault for listening to me. But, I think this is pretty solid advice.