Something I've been noticing lately is that many hitters seem to have a tendency to favor one hand over the other as they swing the bat. Some will be almost all bottom hand, while others are pretty much all top hand. Neither is good, but for different reasons.

If you're using all bottom hand, the result is often hitting weakly to the opposite field — sort of like a golf slice. The bat head never gets delivered powerfully into the hitting zone, and the swing tends to come up short. You can also wind up being a "back slapper."

On the other hand, going almost all top hand tends to make you push the bat into the hitting zone. You can pull the ball, but you don't develop the kind of power you ought to have. You'll also have a tendency to hit "around" the ball, pulling outside pitches that should be going to the opposite field.

The best hitters use both hands in a combination pull-push action. As the body rotates and the back shoulder begins coming around, start pulling the bottom hand in an arc. (Forget taking the knob of the bat to the ball. You don't want to hit the ball with the knob; you want to hit it with the fat part.) This gets the bat accelerating into the hitting zone. 
 
As the ball approaches, the top hand starts moving the head of the bat toward the ball. It fires through the hitting zone, going all the way through extension, and then finishes.

A good drill to learn to use the bottom hand is to fungo by holding the ball in the top hand, tossing it up, and then executing the swing. Starting the bat in the bottom hand tends to force more use of that hand, especially as the top hand usually struggles just to get onto the handle before the swing.

For the top hand/extension, take an old bat to an empty field. Then go through the swing, being sure to pull with the bottom hand first. As you bring the top hand through, try to throw it through the (imaginary) pitcher as far and as hard as you can. A few attempts at that and you'll get the feeling for using the top hand.

Using the two hands in combination, and the proper sequence, will help those weak fly balls turn into fence busters, and those ground balls get through the infield instead of to it.