Mix your shots up!! Easy to say, harder to do. Why? Read on!
Is your shot selection repetitive? Think it through. Is your opponent heading for your shot before you hit it? If so, you must change your shot selection. Part of changing your shot selection is good footwork and mechanics. For example, if you carry your racquet low and want to hit the ball hard you will have to hit cross court to pick up more pace. Therefore your opponent will head off in that direction before you hit the ball. So the first thing to fix are the mechanics of forehand and backhand and then head to footwork.
Back to shot selection. I used to have a rule I used against very good players. “Down the line and shoot bottom board, especially when I have the serve” .
This was very effective as the down the line made the court bigger. If your opponent is comfortable with your shot selection, cross them up! Mix up the setups so they cannot take off before you hit it and be effective.