1. Start running a lot. Not only should you run for endurance, you should be running sprints.
2. Second, do lunges to strengthen your legs. These consist of taking long steps where your back leg knee touches the ground before taken the alternate step.
3. Work on serves. Not only a fore hand, but a nice backhand. Practice by placing a cone where you want your serves to go. Consistency in serves are important.
4. Also, take a group of three. Place one person in the read position and 2 people kneeling on either side of the court by the net. The kneeling people should have about 5-10 birdies a piece. Have them alternately throw birdies into the air of the middle person to hit. This running back and forth with help develop speed.
5. Practice through play, play and play.