I have a master's degree in psychology and took several classes that reveiwed cognitive therapies including training in hypnosis and self hypnosis - and similar therapies such as progressive relaxation and phobia desensitization. We had to go back to the dorms and practice on ourselves and our friends.
I didn't take enough classes in hypnosis to become a hypnotherapist or anything, but I did find it useful. Basically, I've guess I have just been too lazy to use it for dieting. I use it more to get to sleep or for pain relief (I guess because I only think of it when I need instant gratification).
Most hypnosis would be considered light hypnosis. Not many people can be deeply hypnotized (where you would follow bizarre instructions and not remember it when you "wake up"), at least not easily. (Brainwaishing is a type of deep hypnosis, but usually requires extreme sleep and nutritional deprivation and other near torture conditions).
So light hypnosis and self hypnosis can basically be thought of as reminding yourself (or having someone else reminding you) regularly of your goals. Sort of like "talking you into" what you want to do anyway. Say it and think it enough to make it true. That's why not much can really be done in one session for long term behaviors. It has to be repetitive to be successful for long term changes. Any hypnotist that doesn't send you home with a tape to use at home, or ask you to come back for future sessions, can pretty much be written off as a bit of a scam artist. The ones that keep you coming back are also suspect if they don't give you the tape after only a few sessions.
You can do it yourself pretty easily too (there's no need to fear not being able to come out of a trance, because you don't really put yourself into a trance - that's deep hypnoisis). There are good books out there on self hypnosis techniques, and good hypnosis tapes for weight loss also, so if you're interested check out one from the library. But in a nutshell you lie down to soft music or in a quiet room, and before going to sleep you concentrate on relaxing deeply. You visualize being successful at specific tasks. Or you might talk yourself out of cravings for sweets by visualizing tempting treats covered in dirt, or crawling with worms or germs. Basically you "practice" in your imagination doing what you want to do in your daily life.