It's almost a cliché. I hand someone a box of balls to practice their serves. They grab a ball and serve, grab a ball and serve, grab a ball and serve, and so on, all done with the speed and thoughtfulness of firing a machine gun. Then they wonder why their serves aren't any good. There's a lot more to developing great serves than rapid-fire serve practice, where the goal seems to be to empty the box of balls as rapidly as possible. So what should you do differently?
First and foremost, learn the proper way to execute great serves. You can do this by watching players with great serves, or a coach or top player can show you. It's pointless to practice your serves if you don't know how to do them properly.
Once you have at least some idea of what you need to practice, get that box of balls and go to the table. It's generally best done alone; having someone return your serve can be a distraction, especially when you are learning a new serve. (But sometimes you want someone to return your serves, so you can get feedback, and to see how much difficulty they have.)
Grab a ball and get ready to start. You might want to first hold a ball in your fingers (tightly) and practice the actual contact you are going to make with the ball. (But don't rub the sponge into the held ball too hard or you'll damage your sponge.)
Now go into your serving position, and come to a complete stop. The rules actually state that you must start the serve with the ball resting freely on the palm of your stationary free hand--but there's a more important reason to do this than complying with the rules.
This is where you visualize the serve in your head. Don't just grab a ball and mechanically serve it; from now on, never serve a ball without first seeing it done exactly as you want it done, in your head. This is what the top players do. Visualization is one of the best tools in sports, and for serving, it's especially good since there are no outside influences--it's just you and the ball. In your head, see how you swing at the ball, the contact, and the entire trajectory of the serve as you want it.
After you've visualized the serve in your head, go ahead and serve. Don't try to guide it; let the subconscious take over. (You should do this for all table tennis shots.) Let go; you're just an observer. Watch the ball as it leaves your racket. Did it bounce on each side of the table at the spot as you visualized? Did it bounce low to the net as you visualized? Did it go at the speed you visualized? Did it have the spin you visualized? Did it go short or long as you visualized? Am I emphasizing the word visualize enough for you to make clear its importance?
Now visualize the next serve, making corrections for what went wrong in the previous one, and emphasizing the aspects that went right. You are now well on your way to developing great serves. You should also be tired and sweaty pretty soon--serving is a very physical motion. You can't make the ball spin at extremely high speeds if you can't get your racket moving at extremely high speeds, like a whip.
Does any of this sound boring? It shouldn't. If you just grab a ball and serve, grab a ball and serve, grab a ball and serve, that's like working an assembly line at a factory. That's boring. But serving is the trick part of table tennis, and practicing your serves, and all the tricky, deceptive things you can do with them, while revving up and varying the spin, is like practicing a magic trick. That's not boring, and neither should practicing serves.