A couple more ways of saying what essentially has already been said in this thread... but these may stick in your head a but longer. They've helped me.
Pool must be played in the present... not the past nor the future... stay in the present.
The best players pay attention to detail on every shot.
Do not think in the shooting position. All thinking/planning is done before you get down to shoot.
If your mind wanders while you're down shooting, stand up and start all over again. You need your full concentration on every shot... not just the more difficult ones.