Whenever you take a lesson and the instructor has you moving across the baseline during a drill, you will undoubtedly hear him utter the time honored mantra–shuffle back to the middle. This isn't bad advice, as it gets your feet moving between shots and gives you a better workout. But the truth is that during an actual point, the only time you should shuffle back to the middle of the court is when you hit your shot to the middle of the court on your opponent's side. Otherwise you should position yourself to cut down the angle of your opponent's likely return. These two simple rules should help improve your court coverage:
If you hit the ball crosscourt, recover back to within a few feet of the center hash mark. Watch Andre Agassi in a crosscourt exchange and you'll notice that he never moves all the way back to the middle during the rally. He stays a couple of strides toward the sideline because he knows his opponent's best shot back is crosscourt. This saves him energy and gives him more time to set up for his next shot. It also dares his opponent to try the more risky down-the-line option.
If you hit down the line, recover back a few steps past the center hash mark. Opening up the court by hitting down the line means that you may move your opponent more than if you hit crosscourt, but you've also left more room on your side to cover. If you didn't significantly hurt your opponent with your shot, and he has a chance to take a good swing, you should move a few feet past the center hash toward the other side of the court. This way you're in better position to play his best shot, which is crosscourt. If your opponent is late getting to the ball, you don't have to recover that far to the other side because he'll have trouble hitting that shot with much angle.