Over - Under - Middle
Use a series of three pitches to teach them to watch the ball.
The first pitch, the batter swings over the ball.
The second pitch, swings under the ball.
The third pitch the batter hits the ball. Repeat this drill until they can do it every time.
After that, you can really fine tune this:
Pitch 1- just nick the top of the ball.
Pitch 2-Just nick the bottom of the ball.
Pitch 3- Hit it right in the middle of the ball.