tips

1) To make sure that your toss goes to the same place every time, practice your toss with your eyes closed and if you make contact on the strings, it means that your toss was in the right place.

2) The loop is an important key to a fast serve. The loop is when the racket drops behind you as you toss the ball. This lenghtens the swing and therefore increases power.
To a beginner, the loop is very difficult so commonly beginners will tap the racket on their back twice before serving. Eventually, you'll loose this habbit and the loop will be easy.

3) If you're right handed, toss the ball to the one o'clock (slightly to the right). A leftie should toss to 11 o'clock (slightly to the left). This way, you make contact where your racket wants to go.

4) Some professionals will purposely hit a bad first serve and then will be more confident in their second serve. (This is called a double fault). Though it sounds crazy, it works mentally.

5) All professional tennis players know that the power of their serve comes from the movement of their legs, hips and shoulders. Therefore, practice throwing the ball with much hip and arm swing and after that, pick up the racket. The results are incredible.

6) On TV, when watching a tennis match, you'll notice that a player will bounce the ball a few times before a serve. Do this to keep in the mentallity of the serve. It creates consistency through repetition.

7) When you snap your wrist, it's important to get your raquet head through the impact point quickly. If your wrist leads the way, the frame willb e late coming and the ball is sure to go long. So remember to let the head overtake the wrist and hand.

8) Stay relaxed when serving the ball. Try to keep your entire body loose because you use your entire body when serving. Don't tighten up.

9) When you serve from the same spot on the court every time and hit to the same place - and with an identical amount of pace and spin - over and over, you're predictible. You don't want that. So practice changing where you stand, and how much speed and spin you put on the ball.

10) To get more drive out of your legs, pretend to sit when you're serving. (You won't have enough time to go to far down, so you can't lose your balance.)

11) A loose wrist can salvage a lot of bad ball tosses and make your serve more effective. By locking the wrist in place you will make it hard to put spin on the ball at any level. Additionally, a locked wrist can injure your elbow and wrist.
To make sure you don't lock, always keep your pinkie finger, ring finger and middle finger of your raquet hand relaxed. If these fingers are tight, your wrist locks automatically.