One of the most fundamental rules of serving is that you have to be able to serve short. A deep serve may be effective against some players, and up to a certain level, it may always be, but if you cannot serve short, you will always be handicapped against most good players.
A short serve is a serve that, if allowed, would bounce twice on the far side of the table. Because of this, a short serve cannot be looped like a deep serve because the table is in the way. This forces the receiver to reach over the table to return the serve, which can be awkward, especially on the forehand side.
There are many types of short serves, with advantages and disadvantages to each. You can serve very short so that the ball bounces very close to the net. You can serve short so that its second bounce would be near the end-line, which is usually the best option. You can serve sidespin, spinning either right or left, combined with topspin or backspin, or else a pure topspin or backspin serve. You can fake spin but instead serve no-spin. You can serve to the wide angles, to the middle, or anywhere in between. There are endless varieties and you should be able to use most of them.
To serve short takes good touch. Get a bucket of balls and practice alone on a table. If you point the table into a corner, the balls will mostly stay in one spot, so you can practice without long breaks to collect the balls. Practice this until you can control the ball's spin, bounce, and placement.
Start off by serving backspin by brushing the BOTTOM of the ball with an open racket. Try to make the ball barely clear the net. It should bounce close to the net both on your side and your opponent's side. If you do it softly enough, it should bounce several times on the other side; it might even bounce backwards. Contact the ball just above the table level so that it will bounce lower. (Later you will want to be more aggressive with this serve, so that the second bounce is near the opponent's end-line.)
As you learn to control the short backspin serve, try putting sidespin on it by brushing the ball from side to side. Experiment until it feels right. Then practice it until you can do it consistently.
When you can do a short sidespin backspin serve, you're ready to try a short topspin serve. This time contact the ball with your racket going diagonally sideways and up. This action will make the ball pop up and go deep at first, but practice will give you control. Remember to keep the ball low to the net (on ALL serves). Experiment until feels right. Practice until you can put maximum spin on all types of serves and still keep them low and short. (Or fake spin but give no-spin - do this by contacting the ball near the handle, where the racket isn't moving much. You maximize spin by contacting near the tip.)
Generally, all short serves can be classified as either backspin, side-top, or no-spin. You can treat a sidespin-backspin serve as if it were a backspin since both can be pushed - you just have to aim a little to the side against the sidespin-backspin one. A pure sidespin can be treated as a topspin once you get used to it, so unless the serve has backspin, it can be considered a side-top or no-spin serve.
The advantage of the short backspin serve is that it is tricky to attack. The disadvantage is that it can be pushed back heavy or short and is easier to return safely. The advantage of a short side-top serve is that it can be awkward returning it, especially on the forehand, and many times is pushed off the end or popped up. The disadvantage is that it is easier to attack than a short backspin serve. But even if it is attacked, the return of a short side-top serve is easier to deal with because you know in advance the return will almost always be deep. A major advantage of a no-spin serve is that the opponent can't use your spin against you, so the receiver can't push or flip with as much spin. It's also tricky to drop short or keep low, compared to a backspin serve.
You will have to decide which types of spins work best for you. For example, if you like to loop pushes, serve mostly backspin. You will find that certain spins work best against certain players. Another thing to keep in mind is that it is often harder to return a sidespin serve spinning away from you rather than one spinning towards you. For example, a righty's backhand serve is usually more effective to another righty's forehand, and a forehand pendulum serve is often more effective to the backhand.
By serving wide to one side, you make your opponent reach over the table even more but you will also be providing him with the opportunity to hit an extreme angle against you. It is often best to serve to the middle and force the opponent to move both sideways and in, while also taking away the extreme angle. Again, this depends on the opponent as well as your own game.
A short serve can be effective even if done over and over as long as you vary the spin. However, a short serve is most effective when used in conjunction with deep serves, so the receiver has more things to worry about.
Finally, you should watch the good players whenever possible and copy their serves. Don't be afraid to ask questions – most players are glad to give you a lecture on their favorite serve. Best of all, work with a coach who can really help you with your serves.
Placement is extremely important. For example, very short serve can be awkward to receive for some, and draws him over the table, leaving him vulnerable to a deep return. But it can be returned at an extreme angle. See what gives your opponent the most trouble, both placement, depth, and spin.
Most importantly though, get out that bucket of balls and practice!