Great article from Kristen Armstrong on positive self talk in her Mile Markers blog in Runner's World. Here is a quote I love at the end, but I encourage you to read the whole article.
Can you think with me for a second about what we could really do if we stopped telling ourselves that we couldn't? If we changed old habits of self talk, and rephrased our way to victory? I challenge you to listen to the way you describe yourself, out loud and in your head. Stop yourself mid-sentence if you have to, back up, and restate your claim. Words are powerful.