Here are a few formal research studies on hypnosis and weight loss.
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
109 17-67 year olds completed a behavioral treatment for weight management either with or without the addition of hypnosis. Results show that, at the end of the 9-week program, both interventions resulted in significant weight reduction. However, at 8-month and 2-year follow-ups, the hypnosis subjects showed significant additional weight loss, while those in the behavioral-treatment-only group exhibited little further change. More subjects who used hypnosis also achieved and maintained their personal weight goals.
Cochrane, Gordon; Friesen, J., (1986) Hypnotherapy in weight loss treatment
Journal of Consulting and Clinical Psychology. Volume: 54 489-492
Investigated the effects of hypnosis in weight loss for 60 females, at least 20% overweight and not involved in other treatment....hypnosis was more effective than a control group (17 vs. .5 pounds on follow-up).
Kirsch, Irving (1996).
Hypnotic enhancement of cognitive-behavioral weight loss treatments--Another meta-reanalysis.
Journal of Consulting and Clinical Psychology, 64 (3), 517-519.
Averaged across post treatment and follow-up assessment periods, the mean weight loss was 6.00 lbs. (2.72 kg) without hypnosis and 11.83 lbs. (5.37 kg) with hypnosis....At the last assessment period, the mean weight loss was 6.03 lbs. (2.74 kg) without hypnosis and 14.88 lbs. (6.75 kg) with hypnosis... Correlational analyses indicated that the benefits of hypnosis increased substantially over time