The serve is the most important shot in tennis.  If you have a reliable and consistent serve, you're halfway to having a strong game.

Unfortunately, the serve is one of the most difficult shots to master.  Fortunately, it's the only shot you can prepare for in advance without the pressure of a ball heading at you!  So let's look at what goes into a good serve.

The Toss

The most important and underrated aspect of having a good serve is developing a good ball toss.  What many players do is just toss the ball up any old way, figuring that as long as it's generally above their heads, they'll reach up and hit it.  This is a recipe for a bad serve.

The ball toss should be a focused, steady, consistent action.  When tossing the ball, try not to "flip it upwards" with your wrist.  Instead, keep your wrist as still as possible.  Try to toss the ball by "lifting your palm straight upward" like an elevator; pretend you're trying to lift your hand and press your palm flat against the ceiling of a room.  Then, abruptly stop your hand and let go of the ball by simultaneously and quickly opening all of your fingers.  The fingers and wrist should have a minimal role in lifting the ball--most of the ball's momentum upwards should come from your arm.  The elbow should be relaxed, but bent as little as possible.

The idea in the ball toss is to consistently toss the ball to a specific point in relation to your body.  If your toss is all over the place, it will be difficult for you to develop a consistent serve.  Ideally, you should be tossing the ball to a point where, when it is motionless and about to drop downwards, you can strike it comfortably with your racquet.  You should not be tossing the ball so that you're hitting it while it's rising or falling.

Weight Transfer

As you gather yourself for your ball toss, your weight should go to your back foot.  As you toss the ball and move towards it, your weight should transfer forward.  Once you hit the ball, your weight transfer should bring your body forward into the court.

Body Twist

When preparing to hit the serve, your body should be coiled slightly at the waist, storing torsional energy.  As you reach forward and hit the serve, your body should twist and uncoil, releasing that pent-up torsional energy into the serve.  Some players like to accentuate this torsional energy by starting out with their body turned quite a bit away from their opponent (John McEnroe was probably one of the best examples of this style).  Regardless of how dramatically you choose to do it, some amount of body coil is essential to have a good hard serve.

Let It Flow

There are many different styles of serve, but it's generally agreed that no matter what kind of serve you have, the motions associated with it should flow naturally.  A herky-jerky serve, which looks like it's composed of several different motions taped together, is the hallmark of a bad serve.  No matter how you serve, you should try to make your body's movements flow smoothly from one stage of the serve to the next.