If there's one thing I do think about when I'm hitting shots, it's my head. I like to keep it still during my backswing. I get into trouble when I move it down as I take the club to the top.
This is a problem because your head has to move down on your downswing in order to hit through the ball, and if it's already low you're going to hit it fat or, worse yet, lift it up through impact and catch it thin.