Tournament players place an enormous amount of importance on getting the first serve in play. It's easy to see why. In a statistical study of a recent Pete Sampras - Petr Korda match, it was found that if players got their first serve in, their chances of winning a point were 71%. However, if they missed the first serve, their chances dropped to 51%.
Many club level players foolishly consider the first serve as a free opportunity to try for an ace. Their second serve is the one that actually starts the point. Don't get in the habit of trying for aces - it never pays off. Take a second ball and put it in your pocket. You won't need it - the first one is going in. See the ball going in before you hit it. Look into the court and imagine you just hit a deep spinning serve to the backhand. Then do just that.
It takes real concentration to be a consistently good player. Take your time before beginning that first delivery. The person on the other side has to wait for you - so relax. Be like a basketball player - bounce the ball down a few times to set the environment. Say to yourself as you bounce the ball four times, "I can make it." By bouncing the ball before serving you are helping make every serve like the one before.
Start your motion balanced and relaxed. Don't serve until you are ready. Make sure you are totally relaxed as you line up your feet, shoulders, arms and racquet. Consciously make your muscles loose and limp. Think Jell-O before you (slowly) begin the motion.
If, by some freak accident, you happen to miss the first serve, take the same deliberate steps for the second one. There is no such thing as a second serve, so don't treat it any differently. Just hit another first one with a bit more spin.
Remember, you'll never double fault if you always get the first serve in play.