I use this stuff everyday. It works for me. If you believe it won't work, you're probably right. The idea of affirmations is to say it with complete CERTAINTY, with repetitive intensity.
I do it mainly when I'm in the car driving somewhere, or in the shower. I'll repeat over and over "I like myself! I like myself! I really like myself! I like myself a lot!" Over and over again.
Or, "I'm the best!", "I'm so sexy!", "I'm the best salesman!", "I love myself!" You can say anything. Just keep it positive.
Anyways, I'll do this on the way to work or on the way to a hot date. Brian Tracy talks about this in his audio series "The Psychology of Selling". Before any sales presentation, say over and over again in your car "I'm the best! I'm the best! I'm the best salesman!" and just before you get out of the car sit there and visualize. Make a mental movie and visualize the presentation going smoothly. Visualize yourself being calm, relaxed and confident. What this does is programs your subconscious mind to give you the right words to use during the presentation and it will benefit you. Ever had a presentation or interaction go SO smoothly with a girl or whatever and you said everything perfectly and everything went well. You probably thought to yourself, "Wow, that was so easy. I wish everything was like that."
Try it out. =)