Conceive, Achieve, Believe

How do I change my thoughts? I have heard in this program that repeating affirmations do not work. How does one change their thought and actually come to believe it? This months study is on this very subject. Brooke has said something about thinking on purpose. I want to try it for next months project for my 30 day goal, but thinking on purpose sounds the same as affirmations to me. Please tell me how to think on purpose and believe it.