I don't think the OP is about motivation. Do you mean legitimately focusing on the actual exercise that you are doing while you are doing it?
Figure out what your goals are first. A bodybuilder will pay attention to different cues than a powerlifter. Once you establish your goals, come up with no more than two cues that you will focus on during each set.
For instance, when I squat I think about pushing my butt back and my knees out, as this is something I don't do if I don't think about it. Look in the mirror as you lift to make sure you are following through. After a while your motor control will improve and you will perform these cues automatically.