The following comes "Coaching the Mental Game" by H. A. Dorfman:
An athlete must understand a few basic things if he's going to improve his concentration skills. first, he should understand what is possible to control and what is not. It's possible to control one's thoughts, feelings and behavior. We can't control external events, other people's thoughts and deeds, and consequences beyond our behavior.
-The athlete can tell himself what to do in positive terms
-The athlete can focus on the immediate, rather than past or future (The next task -- "now -- is all that can be acted upon.)
-The athlete can focus on his approach, instead of results -- past or future.
It's often easier for an athlete to practice mental skills than physical. he doesn't need a physical environment. he can work on some concentration skills away from the field or arena, sometimes just by sitting in a chair at home.