The first way is to have them "pinch" their elbows together and hit the ball in this position. What this does is eliminate that long front or "lead" arm before the swing. It emphasizes the hands more during the swing which will help shorten the stroke. Try it first off of a tee or soft toss to get the player comfortable.
Also, we have the player's hit off of a tee and place a tall cone or short chair right behind the tee. The idea is to get something that is about up to the hitter's waist or slightly above and make them swing "down" to hit the ball off the tee and not swing "up" and try to "pick" the ball off of the tee. It is really quite challenging and will be immediately rewarding to the hitter. Also, the feedback is instant if they do "loop" and hit the object behind the tee.
Try both of these and I think you will see benefits.
Submitted by: Greg Dennis