Balance, racquet head speed, and the use of your legs to propel you upward toward the ball and into the court are the three biggest points I would suggest to work on. The serve, more than any shot in tennis, is about repetition. Grab a basket, hell, a shopping cart full of balls and serve until the cart is empty and then fill it back up and do it again. Keep serving until you've got a comfortable, balanced service motion that you could repeat in your sleep.