Thought processes are regarded as so powerful nowadays that it is considered that one mind can influence another - e.g. if an excellent swimmer 'hangs around' with a champion for long enough, they will eventually take on that swimmer's habits, training methods, thought processes and behavior, which should eventually bring similar-type results! Great swimmers seem to possess an almost innate ability to handle pressure, control doubt and negative thoughts, and come through when the chips are down. Some of these skills are inborn; often, however, these skills are learned along the way.
These are the attributes most swimmers desire - those intangible qualities that make a true champion. Kieren Perkins was up against huge odds after a bad heat swim in the 1500m at the Atlanta Olympics. He struggled with major nagging doubts and incessant negative thoughts during the 24 hours before the final, so he opted to read a book constantly during this period to cleanse his mind of the nagging doubts. This was a clever piece of innovation, because reading the book prevented any negative thoughts to enter his mind during this crucial period, and kept his mind and body relatively loose and relaxed for the race.
We can learn from this type of strategy. Whenever you feel you are getting 'tied up in knots' from nervousness, providing a distraction can be a clever idea to divert your mind from feeling the pressure. This could include a wide range of possibilities depending on how much time you have - such as a reading a book, simply going outside and having a break from the race atmosphere, listening to some music with headphones (a popular choice), talking or laughing with friends, a computer game, or doing some relaxation exercises such as meditation or visualization.
Distracting yourself from nervousness can simply but effectively re-focus your mind upon something else, taking the focus off the future race and reducing stress (it's amazing how many people actually get nervous about being nervous!) Try this whenever you can feel the physical symptoms of nerves or negative thoughts beginning to bother you.
Perkins also provided an insight to his mental preparation for that Olympic final, and about his thoughts during the race itself. To quote the newspaper article, he said "I visualized exactly what was going to happen tonight" he said. "It's hard to explain, but when you are focused you almost have no thought. Sitting behind the blocks I was 100 per cent focused and I didn't have a single thing in my mind. I knew what I had to do and it was just a matter of letting my instincts take over".
The two techniques Kieren mentioned have been recently discussed in past tips - the enormous benefit of visualizing the perfect swim in your mind, and also getting into the mental state known as 'the zone' by clearing the mind of all thought and simply allowing the body to operate on pure instinct. These are the practices of champions, and they are powerful techniques which work. It's a good idea to master this first in training, in addition to the conscious training sets where you are concentrating on something specific.
One of the ways to becoming a champion is to copy their habits, and find out which ones work best for you. Begin watching and learning from the champions, they are living proof of what may work for you.