Incentives
There's nothing better for a coach than a player who's so self-motivated that the coach's main job is to just keep up with him. You don't need to push a player like that; they are already pushing themselves. But this is rare, and even the most motivated players sometimes need some incentive. Of course success in tournaments and leagues is a primary incentive, but that's long-term. Often players need a more immediate incentive. Here are some I use when I coach.
A primary motivator for all ages is to see how many they can do in a row. At the beginning stages this means things like how many forehands or backhands they can hit in a row, or pushes, or loops against backspin in multiball. Keep it simple, and let them challenge themselves to more and more in a row until it's ingrained.
As they advance, move on to more advanced drills. For example, I've always been a firm believer that one of the key stages to rallying success in matches is to be able to do the following two drills so well you can essentially do them forever. One is the 2-1 drill, where the player does a three-shot sequence: A backhand from the backhand corner, a forehand from the backhand corner, a forehand from the forehand corner, and then repeat. This covers three of the most common moves in table tennis: covering the wide forehand, covering the wide backhand, and the step-around forehand. The other drill is a simple random drill, done either live or with multiball, where the coach or practice partner puts the ball anywhere on the table, and the player has to return each shot consistently, using forehands or backhands. The first is a mobility drill, the second a reaction drill. If you can do both consistently at a good pace, you are ready to rally in matches. The more advanced you are, the faster you do the drills. You can do them live or with multiball.