If you have a player who is suffering from a long, looping swing, then the old standby of the fence drill may be just the prescription for what ails him.
In hitting, the name of the game is batspeed, period. In order to obtain optimal batspeed, the bat must be held with the elbow "in the slot" - in other words, the arms should remain close to the body, allowing the hips and large muscles of the body to provide the power of the swing.
Many times, however, a player will take the old adage of "throwing their hands at the ball" too seriously and end up extending their arms very early in the swing. This not only causes a slow bat - i.e. loss of power - but tends to force a "dip" in the swinging motion, which leads to a high percentage of easy popouts.
To do this drill, set your player facing a fence, slightly more than one bat length away (make sure that there are no fence posts within the player's swing zone). Have the player swing properly through the zone.
If the batter "casts out" with his hands, the bat will contact the fence, causing instant negative feedback. The only way that the drill can be completed successfully is by maintaining a compact swing.
The drill can also be varied by placing the player's back shoulder very near to a fence or net, but the concept remains the same - a proper swing works, while a long swing or a "dip" results in the bat striking the fence.