Humans think around 60,000 thoughts a day, and the quality of these thoughts is becoming increasingly important in a sport where a hundredth of a second is an eternity. We already know many of the problems which are associated with a negative attitude, such as fear, intimidation, lack of belief, intimidation... and so on.
If you still struggle to believe the importance of the mind in swimming, think of this; all of your swimming experience (that is, every lap you have ever swum in your lifetime) is recorded in the archives of your memory bank, and this lies in your subconscious mind. The subconscious happens to be the control-center of all movement, and works just like a computer, and is being constantly programmed by the thoughts, words and statements you choose.
Your results in the pool (the printout) are determined by the quality of the thoughts (the data) which are programmed into your mind. What this all means is this: think negative thoughts before a race, and you will generally get a negative result. Likewise, program your mind positively, and get rid of the mental roadblocks which have been holding you back, and you will get a vastly improved result. Garbage in, garbage out, as they say. The question is: are you satisfied with the results you are getting, or could they improve (a lot)?
Simply by changing the thoughts you think before a race, may help you to change your printout (results) for the future. So how can we do this? Hopefully this answer may help a member of Swim Florida from Fort Myers, who tells me that she and her coach have decided she possesses an 'overactive mind', and wonders if I could help (a common affliction of swimmers).
One of the best ways to overcome negative thoughts before a race is to re-focus the mind onto something positive, by using an affirmation. This is an ancient technique, where you simply repeat a few words over and over again for 5-10 minutes before a race. This does two things; it focuses the mind onto the positive, and it prevents negative thoughts being programmed in before the race. An example of an affirmation could be 'power and speed', or 'perfect rhythm', but you can just as easily make up your own affirmations using your own words. Susie O'Neill did this (mentioned in her book 'Choose to Win') and the greatest boxer of all-time, Muhammad Ali, used "I am the greatest" to great effect.
If you make up your own affirmation, just ensure you keep them short, positive (no words such as 'lose'), and present-tense (e.g. 'I swim like lightning', not 'I will swim like lightning'). Some swimmers even sing a song as an affirmation, and this can also work well. Practice this technique, and see if it makes a difference. I have seen it make massive change and quantum leaps in swimmer's results.

