Try this: If you are trying to hit a forehand groundstroke crosscourt, just line your feet up a little closer to the ball. This alone will help the ball go crosscourt. If you are trying to hit a forehand groundstroke up the line, just line up your feet a little farther away from the ball. This sounds so simple, but it works.
 
One of the problems you might face is this: Suppose you are running to the forehand with the intention of hitting the ball crosscourt. However, you are late with your footwork, forcing you to reach farther to hit the ball. This mistake is a huge reason why so many errors are made in a match. Instead of choosing the direction of your shots by hoping, choose the direction of the ball by your position to the ball. So in this scenario, quit trying to hit the ball where you "wish." Instead, go with the higher percentage shot chosen by your position to the ball. Thus, in this scenario, go with the higher percentage shot down the line, not cross-court. On the other hand, if you want to know where your opponents' groundstrokes are going, notice how their groundstrokes tend to go up the line when they stay away from the ball and cross-court when they line up nearer the ball. Try mastering your choices by your footwork. (Nearer vs. farther from the ball) As your game matures, start paying attention to how your opponents line up to the ball.
 
Have you ever wondered how the pros are able to anticipate correctly most of the time in returning passing shots? They are paying attention to their opponent's position to the ball.
 
Good luck on the courts!