The following passage comes from one of my favorite books, "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.
1. The athlete can tell himself what to do in positive terms.
2. The athlete can focus on the immediate, rather than past or future (The next task -- "the now" -- is all that can be acted upon.
3. The athlete can focus on his approach, instead of results -- past or future.
These simply understandings should be reiterated regularly,, so they become a working philosophy for concentration. Athletes practice physical skills on a daily basis, but they usually just expect mental skills to develop themselves. They don't. The athlete must practice effectively to perform effectively -- physically and mentally.