In college I had a little training in therapeutic hypnosis (not enough to ever use it in practice, except on myself). Hypnosis is related to progressive relaxation, and creative visualization. It can't "make" you do anything, and very few people can be drawn into deep hypnotic trances (where you can be persuaded into not remembering the content of the hypnotic suggestions)
Persuasion is a good way to look at hypnosis. If you hear something enough, you are more likely to believe and act on it, so one time hypnosis tends not to be very effective. Tapes and even positive visualization and "self-talk" can be helpful tools. The advantage to self-hypnosis, is that its easy to learn, and free.
I have been hypnotized for weight loss, (the hypnotist gave me a tape, and I also used self-hypnosis from my college training), but it is something you have to keep up to be helpful, and I got out of the habit too quickly. I do often use it at night for joint pain, muscle cramps and insomnia. I use soft music with no words, or words I can't understand (I like Enya and other Celtic soft vocal music in Gaelic. If I understand the words, I listen to them instead of concentrating on my thoughts and mental images). While listening to the music I try to relax as fully as I can, and I start visualize myself floating on a cloud and the pain going away or falling asleep.
In a nutshell, that's hypnosis, whether you do it yourself or have someone else do it. You relax and while relaxing concentrate and imagine or visualize changing your behavior, condition, and/or image of yourself. Deep relaxation, for some people can be tricky to learn on your own, so renting a good tape or paying for a session or two can be a good investment. If you can find a hypnotherapist who is trained in teaching self-hypnosis, all the better.