Always check out the hypnotist. Being a member of the various hypnotists societies is helpful, but there are devious people everywhere. Make sure that even your oral surgeon is trustworthy. One of the safest things you can do is bring along a friend with you. Any ethical hypnotist will not mind and will make your friend comfortable while he waits.
Hypnotism is a very natural phenomenon.
Highway hypnosis does occur. There are many ways an methods to bring yourself into a hypnotic state. The problem is, once you are there, your subconscious is in control and very is easily distracted. If you have an agenda in mind, don't count on your subconscious to know anything about it. Hypnosis tapes with posthypnotic suggestions to quit smoking, loose weight, remove tension and almost any other need someone may have are available and are fairly effective.