A screen serve is defined as a serve that passes so close to your body that it unfairly screens the ball from the receiver. Three conditions are necessary for a serve to be a screen. (1) The ball has to be "close" to your body. The usual interpretation is that "close" is defined as about 18 inches or within a racquet length (22 inches). (2) The ball must be obscured. That is, if the ball passes close to your body but the receiver can see it anyway, it is not a screen. (3) The receiver must be in good position to see the serve. This means that, if the receiver takes a step in the wrong direction, he loses any right to a screen call. A rule of thumb is that no matter how close the ball comes to the server’s body, if the serve hits the side wall or the back wall within a foot or so of the side wall, it is not a screen serve. The receiver does not have the right to see the ball the whole way or to see the ball hit the front wall.

There should be two lines drawn parallel to the side wall at each end of the service zone, although some courts have only one. The first, 18 inches from the side wall is the "doubles box" where the server’s partner stands in doubles. The second line, not present in all courts, drawn 36 inches from the side wall is the drive serve line. Suppose you hit a drive serve down the line from the right hand (first) position. Neither your body nor your racquet is allowed to go into the drive serve zone. If you violate this rule, it is a fault serve (you get a second serve if it is a first serve, you lose the serve if it is your second serve). If you succeed in hitting a down the line serve with no part of your body or your racquet, crossing the drive serve line and the ball passes through the service box (within 18 inches of the side wall), it is not a screen serve. (If the drive serve lines are not present, you should act as if they were.)

You should hit drive serves to both sides from all three positions. If the serve hits the side wall or goes into the corner, it can’t be a screen. That means that your goal is to get the serve to bounce twice before hitting the side wall near the corner. If it is short, you get a second serve, so plan to ace your opponent on the first serve, or hit a short serve trying. The drive serve does not have to be hit at maximum speed all the time. Hitting a change-up can be very effective.

Here is a link to an excellent analysis, thanks to Jordan Kahn, of what is and what isn't a screen serve:  What is a screen serve? .