This is a simple way to learn how to track your opponent's shot better. First, when you are volleying, imagine where your opponent is on the court, we will call that Point A. Where your opponent's shot would land, would be Point C. More importantly, where your opponent's shot intersects the top of the net is Point B. Point B is where you should try to see the ball. Doing this will prevent you from moving sideways, parallel to the net, or waiting for the ball to come to you. The biggest mistake is waiting for your opponent's shot to cross the net before ever making a move to it. Start seeing the ball while it is still on your opponent's side of the court. Thus, you will try to race your opponent's shot to Point B. This will also help you cut off the extreme angles your opponent might hit.
Many players think that they are too slow or too old to see the ball. I believe from hitting so many groundstrokes, most players learn how to track the ball with their mind and vision to Point C. This works great on a groundstroke, but terrible on a volley.
So, in conclusion, try to remember our ABC's. Start moving to the ball toward Point B while the ball is still on your opponent's side. Remember, good things will always happen when you move FORWARD! Get your mind and vision on Point B.
Good luck on the courts!