It seems like every team has one - a player with a great batting eye who consistently puts the ball in play, but never seems to hit it hard. He teases you with his potential, but for whatever reason, he just can't seem to hit with authority, and most of his at bats result in groundouts and weak singles.
Most of the time, this is due to one of two major flaws - the player is not properly using his hips to generate the force necessary to drive the ball with power, or his wrists and/or forearms are simply too weak and allow much of his generated energy to seep away at the point of contact.
Lance Berkman of the Houston Astros talks about one of the biggest things his father did for him was to have him hit an old tire every day. Every single day, Lance would take fifty swings from the left side, then fifty more swings from the right side (his father was grooming him to be a switch hitter). It paid off, obviously!
To move the tire at all, the player must rotate his hips, using the large leg muscles to generate enough power to drive through the heavy rubber. Likewise, weak wrists and forearms kill the force of the blow, and the tire brely moves.
Some coaches have their players hit stationary tires; while this can provide some help, great care shold be taken to ensure that players do not get into the habit of giving up on their swings, since it is impossible for them to move the fixed object.
Instead, the best idea here is to simply tie an old tire to a large piece of rope, hang it from a tree branch or other suitable overhead object, and have the player take his hacks. Short, tight swings are a must, which the player will quickly learn. Use this drill regularly, and you will certainly see improvement in batting power!