Some thoughts from a retired psychologist that may be of use.
Ever notice how relaxed and loose Efren Reyes seems to be when he plays? That is one of the characteristics of his exceptional level of play. He usually has a laugh or a smile when he misses a silly shot. This is a key concept that is often missed. In any sport one's body must be relaxed and ready get it all together as needed. In a sense you are ignoring all irrelevant information from your body by having it completing relaxed. You can see this loose body in basketball at the free throw line when the player seems to shake themselves loose before attempting the shot. This is the precursor to enhanced focus: Let it all go so I can place all of my attention at one point.
Focus can be described as narrowing ones attention by excluding irrelevant information. It can begin by attempting to visualize the line of travel for the object ball from about six inches behind the OB to the pocket (see Jack Nicklaus' comments on visualization). This line is then visually traced to the contact point and one attempts to visualize the smallest possible contact point throughout the time the player is bending over. This will help exclude other information and one's focus increases with attention to physical detail that is real and imagined.
Increasing focus means that you are excluding other information. The exclusion of irrelevant information so that one can attend to some specific aspect of a task is otherwise known as concentration. Your concentration increases as you trace and then watch the cue ball roll to and make contact with the object ball. The cue stick is left on line and you are, with great intensity, attempting to determine if the cue ball rolled exactly as you wanted.
Verbal ization is disruptive to this process. This is why we make our calculations for hit and cue ball roll before we bend over, One of the things that will improve your concentration is to get the internal dialog out of the way. If you think about it you cannot hum a song you know well and verbalize at the same time. Internal humming and or the use of some thing like “Omm” will help to increase your concentration.
People who practice Yoga know that if you hum th word “Omm” allowing yourself to go up and down the scale, increasing and decreasing the resonance you will find a natural, pleasing sound that suits your body and your temperament. Find this sound and use it internally to stop the dialog and to help increase your concentration.
One way to initialize the level of focus and increased concentration is to use a three phase process: Stand and calculate. When your calculations are done, shake yourself loose (like a basketball player). Use a half crouch to initiate the intense stare at the object ball. This is a signal to your mind to start your personal sound and to stop thinking. As you bend to the shot, exclude all outside information and simply observe your mind and body concentrate on the shot.
Of course you need a good pre-shot routine that you enter automatically. Focus often shifts to stroke mechanics for some people and this is disruptive. To the extent that one can learn to trust their mechanics and never lose sight of the place the cue ball is targeted, focus increases and the body stays relaxed. It is for this reason that planning the cue ball's resulting position is done before one bends over. Once you bend over it is all about narrowing one's attention to exact lines of travel. Basketball players, excellent golfers and pool players are not thinking about mechanics as they make a shot. They are simply looking at or concentrating on the ball.
One of the things that helps one's concentration is knowing there is no aftermath from the resulting shot. Exceptional players simply walk away, shake it off, perhaps laugh and move to the next condition. When the unconscious knows there will be no self criticism following the shot it can concentrate all the more on the shot. Some people might be angry they missed but they are not angry with their self. Jimmy Connors and Earl Strickland come to mind here as do some of the other well known pool players. Anticipation of self recriminations can also effect one's play. The appearance of anger is just that.
Given the foregoing you can see that improving one's ability to concentrate is fairly straight forward. Depending upon where you work and what you life is about, you can create exercises that will help you to rapidly shift into a high state of concentration.
One way to do this is to use traffic red lights. When you come to a red light determine your next move, let's say a left turn. State exactly where you want the car to come out, check traffic, increase your foucs, humm, and observe your body make the turn to exactly the place you want to be.
Better yet, practice at the pool table !
One of the keys to going into and out of a relaxed state of high concentration is to stay loose, relaxed and have a slightly fuzzy feeling between shots. If you are worried about your girlfriend flirting with that guy at the bar, deal with it and get back to the important things in life