I play a lot of basketball and racquetball, as they're both great for your feet and hand eye coordination. Other drills can help as well, such as simply catching a football in distant positions from different heights and velocities.
The two biggest things to understand when you're tracking the ball as a defensive back is your position on the field and understanding that once the ball is in the air you become the receiver. Too many young defensive backs worry about the receiver catching it or what the receiver is doing instead of focusing on what they should be doing. Just go out and make the play yourself, don't worry about him. Know where the ball is and attack it. Put yourself in position to bat it or catch it and make the play.
We used a racquetball and threw it off the wall as hard as we could, then tracked it down with our eyes and feet. Nike has new balls that bounce all sorts of different directions and really help you learn to track the ball and move your feet to react quickly.
You need to understand the receiver and the offense you're facing that week. Know his tendencies and what routes he runs out of what formations. This will help you understand the concepts of what the receiver is going to give you on a given play, and make you that much better defending him.