TIPS:

Bat Angle
Correcting a Weak Swing
Finishing the Swing
Flat-Snap Hitting
Front Toe Closed
Hitting Off Front Leg
Level Swing
Looping
Lunging and Over Striding
On-Deck Preparation
One Swing
Perfecting the Stride
Plate Coverage
Poor Timing
Proper Hitting Goals
Seeing the Ball
Stance vs Swing
Staying Closed
Stepping Out on Pitch
The Two-Strike Swing

Most of these tips come from the book Illustrated Hitting Guide with permission from the author.

Looping

The most serious flaw in the swings of young hitters, especially those with little training, is their tendency to loop. The first thing they want to do when they start their swing is to get the bat head out over the plate. The lock up both elbows and swing away. Older hitters tend to lock the top elbow up (even when they don't need to reach for a pitch), which isn't as bad, but still adds too much loop.

There is a drill that can correct looping called the fence drill. The hitter stands facing a fence (or a large cardboard box or anything soft). He then places the bat perpendicular to his body with the knob against his stomach and the end of the bat about one inch from the fence. Then the hitter gets in his stance, strides and swings. If he is looping, he will hit the fence. He should be instructed to keep his hands about six inches from his body during the initial part of the swing, pointing the knob of the bat toward the pitcher's feet. He also should try to keep the bat head behind his hands until they are in position to hit the ball. The bat head should be behind the hands as it passes by the closest part of the fence.