I'll third what Arne and Jon are saying. When I went back to Rick Welch for a tune-up lesson, he noticed that I was occasionally moving my head during the shot sequence. Sometimes I did; sometimes I didn't. I couldn't tell which shots I was moving my head and which ones I wasn't, other than what he was telling me. After I knew I was doing it, I worked on developing my physical awareness of it so I could know if I was moving my head without someone having to tell me or watching a video of myself. But without having it brought to my attention, I wouldn't have been able to identify this as being a cause of missed shots, because I had no sense of awareness that I was doing it.
Developing your physical sense of awareness applies to all other aspects of the shot sequence, alignment, etc. I would venture to say that once the basics are mastered, a person's sense of awareness determines how skilled a shot he will become. When we say a champion has natural talent, it seems like something that is unobtainable for the rest of us, like trying to grow longer arms. But I believe a large part of that talent is a heightened sense of physical awareness, which we can all work on developing. Of course, another large part is determination and hard work, but that's another story.