Invariably, the players would miss shots now and again, but when the best free throw shooters missed, they tended to attribute their miss to specific technical problems - like "I didn't bend my knees." This lends itself to a more specific goal for the next practice attempt, and a more thoughtful reflection process upon the hit or miss of the subsequent free throw. Far better than saying "I suck" or "What's wrong with me?" or "Crap, I'm never going to get this."

Right, focus on what you can improve to do better.

See also why affirmations don't work:

http://pandawhale.com/post/39729/why-positive-affirmations-dont-work-world-of-psychology

Also, remember the rule of I Don't vs I Can't:

http://pandawhale.com/post/27372/a-scientific-guide-to-saying-no-i-dont-vs-i-cant