Relax your stance, hands and shoulders. You'll have a more fluent swing when you're relaxed, than when you're tense. Watching the ball also helps you learn to hit the ball where it's pitched. Explode your swing into contact with the ball. This is an absolute key to a successful at-bat.
Great hitters fail more than twice as many times than they succeed.
How you deal with your failures determines how successful you will be. When you leave the bench, be ready to swing the bat. Don't be passive and dig yourself a hole. In each at bat, you want the pitcher to feel that you are the hunter and he is the prey. In every RBI situation, you must remember that the pitcher is the one in trouble.