It is a natural tendency to hit balls away from wherever your opponent is on the court. A great plan for the direction of your shot is to use the center strap as an intermediary target, especially at the beginning of the point.
Many pro golfers use this idea when putting. They pick a spot between their ball and the hole and attempt to get the ball started right at it. Applying this thought to tennis will result in a lot of balls hit down the middle when you are in the middle of the court, and some nice angles as you are pulled to one side or the other. You can always hit the ball to the opening as the point progresses.
This concept will help you make a lot more balls at the beginning of the point by giving you more margin for error since the net is at its lowest point at the center strap. It also keeps you from missing balls wide trying to hit down the line to a small area of the court.
Keep thinking "center strap" for direction and pay attention to where your balls end up. You will be happy and surprised with the results!