Service Tips

Here are some things to keep in mind. Don't try to think of them all at the same time. Choose one that applies to you, concentrate on it until it becomes automatic, then move on to the next one.

Ball Toss

Toss the ball higher. Everyone can use a reminder on this one. Any time that your serve consistently hits the net, or lands short, or it seems like you can't angle it, try tossing the ball 1-2 feet higher. Wait an extra moment before starting your swing, rather than pausing with your racquet behind your head.

Second Serve

The only real concern for your second serve is to hit it deep. You might need to practice second serves to feel comfortable hitting them deep. Once you have that down, aim for your opponent's weaker side so that you don't start off the point on the defensive.

Generating Power

If you notice your serve is without zing, or if serving a lot makes your elbow or shoulder hurt, chances are you are trying to get all the power from your arm. Instead, turn your shoulders more (which may mean tossing the ball farther out to your racquet side) and turn your hips more.

If you are standing straight up during the serve, or lean forward throughout the entire motion, try to lean back at the beginning and bring your body forward so that you are leaning forward at the end (or moving towards the net if you intend to volley). This is where your legs and stomach muscles can contribute power.

Be sure to let the service motion end naturally rather than stopping your swing suddenly after hitting the ball.

Exercise

Although you don't want to "muscle" the ball when serving, exercising some body parts can help prevent injuries and can improve your form, especially if you have muscle imbalances that lead you to rely on too much wrist or shoulder. I won't go into the details of the exercises here; consult any reference book on weight training or someone at a gym.

Shoulders
To prevent rotator-cuff problems, do a variety of rotation exercises with small weights (no more than 5 pounds; could be a tin of food). Typically you lie on your side, upper arm parallel to your body, forearm bent at a 90-degree angle, and rotate your shoulder through a forehand or backhand type motion.
Back
Pullovers and pulldowns strengthen the muscles that move the arm forward. A set of light pulldowns is great as a warmup. I find it gets rid of shoulder tightness or soreness that often accompany the first few warmup serves.
Wrist
If you serve with a lot of spin or otherwise rotate your wrist during the serve, a strong forearm is important to prevent soreness or injury in the elbow area. Use a wrist roller (which you can easily make yourself) or do behind-the-back wrist curls with a barbell.

When adding power to your serve, don't change your motion too much. Your fast serve should use essentially the same motion as a regular serve.

Moving

During your serve, you can move but (like in the NBA) too much footwork is against the rules. These kinds of movement are allowed:

Don't shuffle both feet, step on or over the line before hitting the ball, or step past the center line or the applicable side line during your serve. Any of these is a foot fault.