The Serve Statue. The technique of a very young Serena and Venus Williams, in a tape sent to a well known teaching pro.

A really simple activity you can do in your mirror, and never mind what it says about tight clothes or even if you have a racquet.  The main idea is to see yourself and all your little movements, as you transfer your weight from the start position of the service motion to the finish. Pay attention to your back shoulder and where it is in relation to your front shoulder when you release your service toss, and then where it lies in relation to the rest of your body at the finish position.   "Reach up and out...hip over hip & shoulder over shoulder.'