By having mentally rehearsed success in many possible situations, an athlete is much more likely to stay calm, focused and confident no matter what happens on race day.

http://sportsmedicine.about.com/od/sportspsychology/tp/Common-Sports-Psychology-Mistakes.htm

2. Focusing on Mistakes

Many athletes unwittingly mentally rehearse mistakes by repeating them over and over in their minds in an attempt to learn what went wrong. While it's good to understand what mistake was made, by replaying it over and over in your mind you actually make it more likely to keep recurring. Dr. Dahlkoetter explains that the body will follow your mental instructions, unless you rewrite the mistake and visualize yourself doing it correctly. So it's important to visualize yourself 'getting it right' over and over until it feels natural.

3. Over or Under Arousal During Competition

The third big mistake many athletes make on event day is being either over or under aroused. Too much energy, nerves or race-day jitters can result in performance anxiety and interfere with mental focus. In the worst case scenario, too much nervous energy can result in "choking," a decrease in athletic performance due to feeling too much perceived stress.
On the other hand, too little arousal can lead to a mental withdrawal and an "I just don't care" attitude which is very mentally defeating. The right amount of mental arousal is necessary to perform well. Athletes need time, practice and experience to learn what is the appropriate amount of arousal required for them to do their best, but keeping track of arousal levels is helpful.