My coach here at school gave me some advice that totally helped my serve power and consistency.
USE YOUR LEGS!
All your power will come from your legs and heres why:
When you toss that ball up, and then immediately bend your knees in a smooth rhythmic motion, you've coiled up your entire body, much like you should on your forehands and backhands. When you make those legs spring up to get to that ball it's almost like a whip. The handle of the whip is your legs. When you throw that handle up and then bring it down really fast to crack the whip it's the same idea with your serve. Except your body is going down to up, and the whip was going up to down.
The legs spring up and like magic everything else will follow....the last thing being your shoulder to your arm to your wrist to your hand to your racquet. Imagine that. If you can perfect your motion....the only difference will be in your ball toss....which you've gotten advice about that already.
Finally as with everything a human being wants to get better at.....you MUST practice. Get a bucket of balls and serve a bucket or two when you can....it wont take more than 45 minutes if you're intent on what your doing. Imagine that service motion. Soon enough you'll start to know when you've screwed up your motion before the ball crosses the net. It'll just become second nature. Much like a cheetah must hunt to eat.....a tennis player must serve to win.