Endurance training:
run a lot and build up your stamina by pushing yourself, if you have good lung capacity and cardio you'll be much better off in long matches and points
Build lean muscle:
Do lots of low-weight, high-rep weight lifting. This means lift lower weight sizes, but do them a lot(instead of doing 20 pounds 10 times, do 12 pounds 25 times)You won't bulk up, but you'll get good lean muscle that is good for agility sports like tennis.
Work out your core area as well
That's what I do, those 3 things will help you a lot in tennis. Also build up strength and good motion in your shoulder area, that's always a problem area for me in serving(get sore upper back/shoulder due to lack of good movement in the off season)
This post was edited by BlackKnightsOfFistingPower on Feb 21 2011 09:54pm