I think it is important to consider something here as well. You do NOT need to believe that it will work in order for it to work. What you do need however is a willingness to give it a try and see what happens. I've had a number of clients who who tried so many other therapies in the past that did not work so when it came to hypnosis they also did not believe it would work. However, one was willing to give it a chance out of desperation.
Hypnosis is not placebo or magic. Neuroscientists out of Columbia University discovered last year that hypnosis works because it completely and automatically override's the subject's top down processing. Researchers were able to able to completely abolish the Stroop Effect in the brain. They gave suggestions while the participants were in hypnosis that cancelled out this effect then put them in an FMRI and administered a Stroop test, the parts of the brain that should have lit up in this kind of test did not at all.
This type of result is not linked with belief in hypnosis nor the suggestions, this was an immediate and automatic effect.
Hypnosis does not function through belief, it functions through information processing. This is not to say that belief is not important. Hey, if you believe a medicine isn't going to work strongly enough then it won't. When it comes to therapeutic practice it makes my life much harder when I have a client who is certain they are not going to change. I will fire clients like this because they're motivation is not there which means the likelihood of them taking part in the therapeutic process is minimal.