Learning how to disguise your serve in order to catch your opponent by surprise can be a tricky proposition. It is often difficult for us, as players, to practice an efficient disguise because, prior to striking the ball, we know exactly where we intend the ball to go. As a result, we often give the serve away by adjusting the toss and preparation just slightly in order to hit the target.
One way to practice the disguise - that is, having the preparation be exactly the same for every serve - is to practice the serve while the coach is behind you calling out the target in the air. For example, set up three different targets: wide; middle; and T. When the ball reaches the apex of your toss, the coach calls out "wide" or "jam" or "T". S/he tries to mix up the signals so that no pattern is established. Once you get the hang of it, the player can practice the slice, top-spin, flat or kick-serve in the same fashion.
Practice disguising your serve and you will have the element of surprise on your side. After mastering the basic drills, do it under pressure. For example, put something on the line where if you are within 3 feet of the target you run a brief sprint or do some kangaroo jumps. This will not only get the heart racing but will make serving practice competitive and exciting.