The New Scientist (Oct 1992. Robert Mathews. Issue 1845) has undertaken research into success rates of different methods of stopping smoking, and found that hypnotherapy was the most successful.
Here is a summary of the findings:
88% success with hypnotherapy (after a 1 year follow up),
60% using relaxation methods,
24% using acupuncture,
10% using nicotine gum...
...and a mere 6% using their willpower alone.
As with all Hypnotherapeutic interventions, the client must WANT the therapy for themselves,in order for a positive outcome.