Originally Posted by Cheetah
Another way to do this is to stand with both feet pointed towards the net or maybe just slightly angled off the net like how they are at contact and hold that base and practice serving like that. That will help with coordinating balance and the rotation together and it's a good way to feel the arm being thrown around and up from the body rotation. It's pretty good. give it a try.
This is an old drill and it works well. Teaches you to rotate shoulder back and then foward. You keep the legs relaxed but stationary - no knee bend or explosion up. Basically, you are serving with rotation of the shoulders and hips may rotate a bit too. I do this sometimes when warming up. Once you get comfortable with a smooth continuous rotation, you can really get pretty good pace just using upper body rotation. I have seen video of Paul Annacone somewhere teaching this drill.