First of all, the research is pretty clear that medical hypnosis supports weight loss and more importantly supports long term weight loss.

Study One:
Bolocofsky, David N.; Spinler, Dwayne; Coulthard-Morris, Linda (1985). Effectiveness of hypnosis as an adjunct to behavioral weight management. Journal of Clinical Psychology, 41 (1), 35-41.

The first study, published in the Journal of Clinical Psychology, divided 109 people into two groups for a 9 week study. The first group were treated with changes in diet and exercise habits (the only way to lose weight) without the addition of hypnosis. The second group was given the same diet and exercise treatment and was also given hypnosis for reinforcement.
After 9 weeks, not surprisingly, both groups lost weight due to the changes in diet and exercise while under supervision.

What happened after the 8 month and 2-year follow-ups might surprise you. The group that did not have hypnosis did not lose anymore weight and in fact most gained most of their weight back.
The hypnosis group, however, continued to lose weight during both intervals and the studies showed that far more people in the hypnosis group met their long term weight loss goals.
The take home is that the hypnosis group continued the eating and exercise habits learned in the experiment while the non hypnosis group did not.

There is no magic pill for weight loss and you simply have to follow a healthy eating and exercise plan - from this study it is clear that hypnosis supports diet and exercise adherence.

Study Two:
Cochrane, Gordon; Friesen, J. (1986). Hypnotherapy in weight loss treatment. Journal of Consulting and Clinical Psychology, 54, 489-492.

The next study, published in the Journal of Consulting and Clinical Psychology, investigated the effects of hypnosis in weight loss for 60 females at least 20% overweight and not involved in other treatment. The researchers interviewed each participant this time about their family background, educational background, and their belief as to their hypnotizability.
The group was split into hypnosis and non hypnosis groups with both groups being given weight loss behavioral treatments and counseling. The results were very interesting. The group using hypnosis lost an average of 17 pounds at the 6 month follow up. The group that did not receive hypnosis only lost a half of a pound. They also found that "hypnotizability" and social and educational backgrounds had nothing to do with the results.

Study Three:
Kirsch, Irving (1996). Hypnotic enhancement of cognitive-behavioral weight loss treatments--Another meta-reanalysis. Journal of Consulting and Clinical Psychology, 64 (3), 517-519.
In this meta-analysis, (which is when researchers look across multiple studies), the results of the two studies I just mentioned and other studies were confirmed.
In the studies of hypnosis versus no hypnosis the average short term weight loss was 6 pounds for those not using hypnosis and 11.83 pounds with hypnosis.

Even more importantly, over the long term the average weight loss without hypnosis remained at 6 pounds, but with hypnosis it jumped to 14.88 pounds - indicating again that the effects of hypnosis seem to compound over time.

What is impressive about these results is that doing hypnosis under controlled conditions is extremely difficult, and to have the research indicate that people are losing anywhere from 2 to 17 times more weight under hypnosis than when they do not use it clearly indicates that hypnosis should be used by anyone who is serious about weight loss.