Many players go through this series of events as the opponent is hitting the ball toward them:

1. Opponent begins forward swing.
2. I wait to see where the ball is going.
3. Opponent hits ball.
4. The ball is coming toward me.
5. Do I have to move?
6. Yes, I have to move!
7. I prepare to move.
8. I move.

More advanced players go through this series of events:

1. I prepare to move.
2. Opponent begins forward swing.
3. From forward swing I see where the ball is going.
4. I move.

Which do you think is more efficient? The two most important things from this are that you should get into the habit of trying to react to where the opponent is hitting the ball as he begins his forward swing, and that you should assume you have to move.