The first thing you need to know about hitting a baseball are the two C's.

Comfortable
Confident

By the time a baseball player gets to the plate it's too late to make any major mechanical adjustments to his swing. The ball simply get's the plate too fast for the conscience mind to manage the action of swinging the bat. Therefore it's critical the player spend ample time working on the swing away from the batter's box. Once that batter steps into the batter's box however there are some things which can be done to make your more or less successful.