Blame It On the Toss
In all of sports, there isn't a longer or more complicated motion than the serve. Except for your menacing glance across the net at the beginning, everything that comes after is vulnerable to problems, and they can twist the serve into a tangled mess of double faults.
The good news is that most service problems can be traced back to the toss, that seemingly simple three-foot lift of the ball into the air. Fix the toss, make it consistently good, and most service problems disappear. Faults that go long, for example, are usually the result of tosses too far behind you. And, you guessed it, faults into the net come from tosses too low and too far in front of you. The height of your toss is controlled by your tossing hand; the forwardness of your toss is controlled by your weight shift.
Tossing too far behind you is usually the result of a weight shift in the wrong direction or no weight shift, at all. Your weight should begin on your back leg, and as your tossing hand rises to release the ball, it should draw your weight from the back leg to the front, exactly the weight shift you use when throwing a ball. If you shift, rather, from front to back, or don't shift at all, the toss goes behind your head and forces contact with the ball's back. Out it flies. However, if you toss too early, the ball goes forward but without the necessary height. On a low toss, you hit too much of the ball's top and smack it into the net. Your tossing hand must lift to its full extension over your head to correct this. Lift your eyes all the way to the sky before you toss and your hand will follow. Remember also, the toss is a lift, not a throw. Only your shoulder should move, not your elbow or wrist.
c Keith Shein