Tennis, like everything else in life, has a set of rules that make life easier when followed but complicate matters when not. I call this set of rules: the laws of the battle. They describe the ideal response to any situation that a player may encounter on the court, just like a chess manual describing the best move for every position on the board. They are the theory behind the tactical aspect of the game, and the first step towards helping players understand how to use their weapons effectively, or in other words understand where to hit their shots to have the best chance of winning the point.
Lets take a look at laws 3, 4 and 5 of the first serve in singles:
3) Look to move into the court to attack a short return after your first serve.
Remember that the goal of your first serve is to force a weak return from the opponent, so make sure that you are expecting a short ball. There is nothing worse than hitting a great serve that the opponent barely hits over the net and then not being ready to attack the second shot because you were not ready to move into the court fast enough. Next time you watch a professional tennis match count how many points are won by the server the next shot after the serve.
4) Vary the location, pace and spin. Use the serve to the body.
Even a very hard serve will not be very effective if it is hit to the same spot every time. The key to great serving is to keep the opponent guessing. Use different spins, speeds and targets.The serve to the body is a great option that is not used enough. A good serve to the body is a great, especially on key points against an opponent who has been returning very well.
5) Use the slice serve to hit wide or to the ‘T’.
A slice serve bounces away from the player. The additional spin allows you to land the serve closer to the net when serving wide, thus pulling the opponent farther off the court. This is an especially effective serve against players with extreme grips or left-handers.
From the book "Developing High Performance Tennis Players"