Originally Posted by LeeD
Head for the center of your opponent's possible return angles. You dno't need to be there, only headed there in time to cover the other side.
This is the textbook answer and I think it should be modified a little to give bias to what is your opponents highest percentage shot and lowest percentage shot. So, if on one side the shot is way more difficult than the other, give more priority to his better percentage shot. In this way you encourage him to go for more risky shots.