Cue ball concentration
Set up the cue ball for a straight in shot on the first ball in the string. You are striving for perfect cue ball kill. If the cue ball moves left or right you did not stroke straight (that's assuming you set the shot up straight). After making the first shot get the cue ball and set up for the next ball. The idea behind concentrating on cue ball kill is that it will get you to think about the cue ball not the object ball.