I've always been skeptical of the self-help advice to use affirmations such as, "I am thin," if you are overweight, or "I make a million a year," when you're barely able to cover your rent. So I was glad to see that a study published in the journal Psychological Science, confirms this.
The author of the report said repeating positive statements worked only if it reinforced what the person already genuinely believed to be true. If the person knew it wasn't true, the counter-argument ("Actually, I'm fat," or "A million? I'm broke!") overwhelms the positive.
So what to do? One method that I think does work is to figure out what behaviour would move you in the direction of your desired outcome, then visualize yourself doing it before you do it for real.
For instance, if you want to lose weight and have been invited to a friend's place for dinner, picture yourself saying "no, thank you" to the dessert. It works even better if you imagine it twice: first as though you're watching yourself on a movie screen, then through your own eyes.