The American Heritage Dictionary says that to concentrate is "to pay close attention;" to focus is "to concentrate on." While focus has become the more popular term within baseball today, focus by definition involves concentration. If a person has a highly developed ability to concentrate, he will be able to focus well on the task before him responding very little to any possible distractions.
The most common distraction that affects a pitcher's concentration is self-consciousness. Rather than paying attention to his task, the pitcher spends more time worrying about himself, his doubts and his fears. Other common distractions can be the actions or comments of coaches, parents, or other players. All of these relate back to a pitcher's level of self-consciousness. Therefore, the task of the pitcher is to redirect his thoughts onto his responsibilities as a pitcher. Doing so will reduce the strength of distractions and help to get rid of anxiety.
Contrary to popular belief, concentration is a skill and something that every player can improve upon. So, how can I get better at concentrating, truly focus on pitching, and put distractions out of my head? Practice! Just as in other physical and mental skills, the best place to work on concentration is at practice. As you go through drills and exercises, consciously attempt to keep your mind thinking about the skill you are practicing. More specifically, when practicing pitching, focus your mind on the specific movement you are working on (landing position, for example). If there is no single section of your motion to be worked on, maintain a disciplined focus on the target. "Concentration is not just looking at a target, but seeing it."
This is a tough task because your mind will naturally tend to wander. When you catch yourself thinking about tomorrow's band recital, tonight's Simpson's episode, or the yelling and screaming of teammates and coaches, stop yourself and turn your focus back onto your responsibility. By making sure you are intensely focused during a practice activity, and by monitoring yourself and that focus, you can improve your ability to have selective attention and increase the length of that attention. In other words… concentration is a skill, and you can get better at it through a disciplined approach and practice!
Here is a drill from H.A. Dorfman's, The Mental ABC's of Pitching: Draw a grid with 100 small squares and make photocopies. Fill in one grid with the numbers 00 through 99, using double digits (01, 02...). Next, while timing the activity, cross out the numbers in numerical order. Fight frustration with a conscious effort to refocus. Change the number locations on the grid to avoid memorization. When your times grow noticeably shorter, put on the radio or a CD player. Start with classical music and progress to music with lyrics. Then raise the volume, or any other variations you can think of to increase the challenge.