In order to get rotational strength when hitting a baseball, you have to get the power in your hips. When you swing, turn your hips and then bring your hands to follow through for extra power.
My recommendation, and this is a technique that I used to use myself, is to go into the woods with an axe and fell some trees. Swinging an axe and a baseball bat are similar motions and the repetitive motion should increase your rotational strength.
