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.
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.
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.
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.
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.
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.
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.