"I'm Freaking out man!"

At one time or another your are going to experience a race that goes really well and a race that doesn't happen the way you had liked it too.  Often times the only difference between those two races is what happened right before or what your excitement level was.  The really important question to ask is how do I continue to repeat the good race and never have a bad race again.

The bell curve
Every swimmer has their own bell curve graph of arousal vs. performance

The idea is that there is a specific range of arousal (anxiety, scaredness, pumpedness, etc) that an athlete performs best at and all other levels they perform worse.  Once you have identified the level of arousal for your peak performance you have to find a method of reproducing that level for every race.

Here are some things that might help to keep your arousal low if that is the kind of athlete you are.  
  - Close your eyes and visualize your race being swam perfectly, confidently, easily.
  - Have a friend with you to talk to so your thoughts are not dwelling on the race.
  - Pick a skill in the race to focus on like streamlining the best you ever have, or kicking off the wall better than ever before.

Visualizing your Race
Visualizing can also help eliminate "brain fart mistakes" like swimming the wrong stroke, or one hand touches.  When you visualize your race make sure to use all five senses and be as detailed as possible.  This will increase the effectiveness of your visualization.