I do hypnosis clinically in my office.
Hypnosis can be used to block memory, but only if the hypnotized person agrees. In the same way, a person can be talked into almost anything, but only if they were open to doing it anyhow. It's simply a matter of who is willing to be seduced. - And, like anyone else, hypnotized people my do something questionable today and regret it tomorrow.
For a person seriously interested in knowing what happened at a hypnotic session, even if the hypnotist is quite expert at blocking recall, the memories will eventually emerge.
As an psychoanalytic intern I went through about 200 hours of "training therapy" in various settings, including with a hypnotherapist. After hearing about some of my experiences twenty years previously, days when I was young, foolish and trying to live as a hippie, this well-meaning therapist was evidently horrified at the idea that I was a drug user. So she went to very great lengths to make sure that first, she would make it impossible for me to use drugs, and second, that I would not remember the session. It would have made better sense if she had looked at a caendar and observed that SHE WAS 20 YEARS TOO LATE. I had long ago learned that drugs make a very poor substitute for real life and had stopped the excesses of my earlier days.
I became suspicious when I started to find it awkward to take an antihistamine, an aspirin or Tylenol. These are certainly drugs. They led to burping and stomach upset. It wan't much better when I took analgesics like codeine for my arthritis - same problem. It took almost three weeks to fully decypher the suggestions that she had given me, but they were all there, and my mind was aware of everything on some level, and had been aware all along.
So, the first lesson is that even an expert cannot hide a memory with hypnosis.
Second, even though an expert seducer can convince a person to do something, the memory and responsibility for the event are always available, and will eventually will be available for review. That means that even if Suzy of Johnny agreed to a sexual act last week, whether because of being hypnotized and seduced, or merely because of being horney, this week might bring a change of mind and complaints. (It happens anyway in everyday life.) To claim seduction while in a therapy session would be a felony charge for a licensed psychologist in California, and could lead to equivalent charges for a hypnotist, in addition to cancellation of professional liability insurance.
Third, messing with people's heads in this manner is extremely unethical. It amounts changing the hypnotist's role from "Therapist" to "The Rapist". To give any suggestion or to approach any topic other than that requested by the client is unethical and will result in all major organizations rescinding certification of the hypnotist, making insurance impossible, and essentially blackballing that practitioner from other practices. Hypnosis is brain surgery. Those who cannot treat it in that manner don't need to practice in the public sector.
Now - given all that - if you want to work with a hypnotist, my suggestion is to look at their professional memberships, and see what that group believes. For example, I belong to the IARRT (past life therapist organization) and IHF (liberal hypnotherapy organization) both of which have high ethical standards. I've doing this for more than 20 years. These are marks of someone who is at least serious about what they do.
A professional hypnotist should have some idea about how to approach the job you want done. If it's past life, they will have experience in that area. Non-professionals, like friends, can be effective as well, but tend to be limited in resources and techniques. I have a script on my web site that newcomers and amateurs can use - it's the one I use when I work. And when I do a regression, I offer a video record of the session - this solves all problems of trying to remember what went on, and it eliminates issues about improprieties.
When selecting a hypnotherapist, it is useful to know what the basic options are. I suggest that there are three general levels: (1) For cases of redirection and simple motivation, NLP (neuro-linguistic programming) is sufficient. NLP is an associative conditioning technique that works on the superficial levels of attitude. (2) For cases involving habits and confusion, suggestive methods are generally adequate. These allow redefinition of a situation, altering payoffs so that habits stop being reinforced, and sticking inhibitory behaviors between triggers and the habitual response, so that some other action occurs. (3) Very deeply buried material is available through hypno-analysis, a basic regression approach. Regression simply means moving the mind back to a prior event, whether in this life or some previous lifetime. The idea is that when material is examined, the frightening aspects vanish and it ceases to have power over us, and finally it just becomes old business and we can let it go.
For past lives, you need someone with a background in regression, or willingness to play with it as the two of you work out a routine. It's not dangerous, providing that everyone is willing to be ethical. In fact, I think its pretty good fun.
dave