Know your game and your limits.
So many people try to hit as hard as Federer, or put as much topspin on the ball as Nadal. We are not pros, and the more we accept our limitations the better we become at the game by not trying to smack the ball and making so many errors.
If you keep your head still but swing as fast as you can, the ball will never go in; If you work solely on footwork, but you brush up on the ball like a monster to produce a shank, you will never be consistent; Even if you slow down your stroke to negate the first two possibilities, you will expect much more to happen than can actually occur. This is why I believe knowing your limitations (which includes knowing who you can and can't beat) is the best advice in one sentence. Even if the student doesn't understand it at first, he/she will eventually get it.
I am an unpredictable player. Even I surprise myself with some of the shots I make.