Basketball is a game of chess, and the ones who get caught playing checkers are routinely beat over and over again. You have to be thinking one step ahead at all times. Is there a screen coming? What kind of screen is it? Is he about to drive or shoot – and from where? Where is my next help responsibility? Anticipation is a key ingredient to success on either end of the floor. If you can couple anticipation with technique you will have success guarding. It will look like you are in two places, but you know that you are just thinking a step ahead.