Do the thrower's ten to help prevent overuse injuries to your shoulder, elbow an wrist from all your serve practice:
You can use a theraband flexbar at your desk: Thera-Band FlexBar Video Demonstration http://www.youtube.com/watch?v=zB3TVb8a5mk
The serve is powered by the whole body in a kinetic chain fashion, with the power rippling up from your leg push off, through a dynamic uncoiling/reversal of the bow shape of your core, and finally up into the arm.
Like any chain, it is only as strong as it's weakest link.
For a total body workout consider:
Tennis Weight Training - Exercises of Weight Training for Tennis http://optimumtennis.net/tennis-weight-training.htm
Many prefer to use these long summer evenings to play more tennis, and concentrate on starting a more rigorous strengthening program for the fall/winter.
But here is an approach for a more prolonged weight training program:
The Elite Approach to Tennis Strength Training http://www.sport-fitness-advisor.com...-training.html