Many tennis players look at the professionals and think they are hitting down on their serves. In fact, it is the just the opposite. One reason a player believes he/she can hit down on the serve is because he/she can see the service line through the net. If you imagine that the net were solid, you could not even see the service line or any other lines on the other side. Because of this, the net would appear much more formidable. As a result, you would be forced to hit up on the serve.
Imagine there is another 5 foot net on the service line on your side of the court. Your serve would have to leave your racket, go up over the imaginary 5 foot net and then go over the 3 foot net.
I believe a great mistake players make on their service is throwing their toss too far out or away from them. A good habit is to throw the toss up where you can see the bottom of the ball, not the back of the ball.
Many players think a pro is jumping into the court. I believe it is better to jump straight up. As you go up and off the ground, it forces your spine to straighten, not bend over, and it forces you to use your legs and adds different forces to the ball. As your body rises, it becomes weightless. While your body becomes weightless your racket is projecting forward and up. As a result, you will see all great right-handed servers enter the court with their left foot and all great left-handed players enter the court with their right foot.
Remember, a serve that passes over the net low tends to bounce low. A serve that passes higher over the net will bounce high. If you do not have an "Andy Roddick" serve, you should sacrifice a little power for height over the net and depth into the box.
If you need more time to go to the net, hit your serve higher over the net and deep into the box. When you are returning, would you prefer a waist-high ball or a ball bouncing over your shoulder? Remember, you opponent likes the waist-high ball and hates the ball bouncing up high.
If you are learning to hit "up" on the serve, do not forget the 3 things all pros do to keep the ball in: leave your toss hand up as long as possible, come off the ground with both feet and accelerate your racket-head speed.
Good luck on the courts!