One of the big mistakes players at the intermediate/advanced levels make is to serve short over and over against an opponent who doesn't attack long serves. I've watched national championships lost because a server insisted on serving short against a receiver who was very comfortable returning short serves but had great difficulty against deep serves. At the lower levels, and often well into the advanced levels, players often serve long because, even though serves are generally easier to attack (usually with a loop), they have major advantages as well. Done well, a deep serve turns the receiver into a puppet, with the server holding the strings.
The main advantage of a short, low serve is that it is difficult to attack, since the table is in the way, making it difficult or impossible to loop. It also brings the opponent in over the table, leaving him vulnerable to a deep attack.
However, a short serve also allows the opponent to rush you with a quick, angled flip or push, or to drop the ball short. The shortness of your serve means the opponent can contact the ball much closer to you than off a long serve (so you have less time to react), as well as giving him a wider angle. A receiver can also drop a short serve short (so that, given the chance, it would bounce twice on your side of the table), stopping the server from looping as well as catching him off guard if he was looking to attack a deep return.
Serving long takes away these advantages for the receiver, and if he can't attack the deep serve effectively, then there's little reason to serve short too often. Instead, serve as deep as possible, knowing you'll have more time to react to his shot and that his returns will have less angle available. Deep serves should go very deep, with the bounce somewhere near the receiver's end-line. Serves where the ball bounces more toward the middle of the table (depth-wise) are not nearly as effective - they are more easily attacked, they don't jam or force the opponent off the table, and they allow the receiver to hit the ball quicker and with better angles than off a truly long serve.
This doesn't mean you serve long every time, allowing the receiver to hang back, waiting for the deep serve. Mix in short serves so he has to watch for those as well, making the deep serve even more effective. A mixture of long serves to the backhand and short to the forehand are especially effective.
If you like to serve backspin and loop against pushed returns, and your opponent doesn't loop against deep backspin serves, why serve short? Serve long, giving yourself more time to react to the receiver's push, while also cutting down on his possible angled returns.
There is one variation of long serve that doesn't go very long - the half-long serve (sometimes called a tweeny serve). With these serves the second bounce on the receiver's side, if given the chance, would go just barely off the end (or, as a confusing variation, the second bounce would just barely hit the table near the edge). These are difficult to attack effectively, they don't allow the receiver the quick, angled returns they could off a shorter serve, and they are almost impossible to drop short.
Placement is also important. Often players serve right into an opponent's waiting forehand or backhand. Instead, serve at either extreme angles or to the middle. If you serve at extreme angles and very deep, the receiver will have to move to the ball, and it's not easy for him to make a strong return off a deep ball when he has to run it down. When you serve to the middle, it should be right at the receiver's playing elbow, the mid-point between forehand and backhand, and very fast, forcing the receiver to make a quick judgment on whether to use forehand or backhand, and then moving to cover it - not an easy return.
Deep serves should focus on either speed or spin. Speed serves rush the opponent, forcing weak returns. They are often most effective if served dead, i.e. spinless or even with a very light backspin, forcing many receivers to put the ball in the net (especially when served to the middle), or to over-compensate and go off the end. Spin serves can be served either fast (so they rush the opponent, but allow him to use your own speed against you with a counter-hit) or slow (so they break more dramatically but give the receiver more time to react), and should break as they approach the receiver, so the receiver has to both move to reach the ball and have to compensate for the spin. Most often you want the breaking serves to break away from the receiver, especially if you can serve deep into a receiver's backhand so it breaks away from him, making him reach for it. (Most players have less range on the backhand, so forcing them to chase after a breaking serve here is especially effective.) When reaching for a ball, receivers often lower the racket, and so they lift the ball high (giving you an easy smash) or off the end.
So develop those deep serves as a powerful weapon, along with short serves. The only thing more fun than turning a receiver into a puppet as you pull the strings, moving him around at will with your varied serves, is to win national championships with them. You might not do the latter, but you can be a serving puppeteer.