Are you the type of player who can serve his way out of trouble but have some difficulties when it comes to breaking your opponent's serve?! If so, this drill may be helpful. One of the keys to successful returns is to rely on the feet more than the racket. The return should be attacked and, yet, too many players rely on the racket rather than the shoes to get them to the ball. They are reactive as opposed to being proactive. The problem with this attitude is that the further back you intend to make contact the greater the distance that you have to cover. This distance is not only measured left-to-right but also up-and-down (since a good server's ball can really bounce high if you let it).
To assist you with the concept of attacking the ball with your feet, put the racket down and grab a baseball glove (or two - one for each hand). Have your coach or practice partner mix up serves keeping them as far away from you as possible. Have her slide serves out wide; kick them to your backhand or hammer them into your belly button. Your goal is to move forward and catch as many balls around your waist as possible. By moving forward (like a team handball goalie: video below), you will learn to cut off angles and to do less with the racket and more with the feet. In the process, you will learn how to adjust your positioning by being light but quick and explosive.
When you master 10-15 good catches in a row, switch things up by alternating regular returns with catches. Transfer the skills from the catching to the returning and learn to use more of your opponent's pace and angles against her. By being a proactive returner, you will put yourself in a better position for break opportunities.