When you watch top level players serve, notice that all of them go through some sort of pre-serve ritual before they toss the ball up to hit their serve. They are never in a rush to serve, and they never just walk up to the baseline and serve, without fully preparing, like most club players and most juniors do.
Remember, the serve is the only shot in tennis that you don't hit from a ball coming towards you – the ball is sitting in your hand, and you don't have to hit it until you're good and ready. Don't rush to serve on your opponent's account – he can't do anything but wait. You have a real advantage having total control over the shot – you get to, basically, throw the ball up in the air and smash it at your opponent. That's a huge advantage.
Toss the ball up and hit an ace!!
This ritual gives you time to relax, loosen up, and physically and mentally prepare to hit the most important shot in tennis. This is when you plan out a point – it is when you decide what kind of serve you will hit, where you will place it, and what you will do after hitting it. Remember, hurrying up to serve usually means hurrying up to lose the point.