More Tips:

http://tennis.about.com/od/instruction/u/instruction.htm

Move quicker on overheads

Often, players immediately begin backpedaling when a lob goes over their head.  Instead, turn sideways to the net as soon as the lob is hit and "scratch your back" with your racquet.  Now, you can sidestep or run toward the baseline looking over your shoulder at the ball.  This is faster than backpedaling, plus you are ready to swing at the ball.  If you retreat too far, you can always run back in and take the ball out of the air or let it bounce.

Keep your shot from going wide

If your shots tend to drift wide when hitting down the line, try "aiming" further away from the line. Try to hit your shot halfway between the center and the sideline and you may find your shots going right down the line like you wanted.

Add some zip to your forehand

Try "catching" the racquet with your off-hand on the follow-through.  Don't keep your arm at your side.  Catching the racquet allows your body to rotate more freely and naturally, thus enabling more racquet speed and more "zip" to your shot.