Hi Military
Deborah's suggestion is a good one. There are also various books about this. You might take a look in the books section here. I recommend anything by Michael Newton and I read Exploring Reincnarnation by Hans Ten Dam a while back, which I thought gave a very good overview of the whole 'thang', from the ancient literature on the subject to Edgar Cayce, right up to the current day, Dr Stevenson's work, various new age beliefs and so on. It is a good 'primer' on the whole topic. Dr Ten Dam is a past life therapist himself and talks about the process.
Regressionists use hypnotism - which means putting someone into deep state of relaxation usually just using suggestion and the soothing tone of the voice (they don't usually swing watches any more). That is not the same as putting someone to sleep. Usually the person in the trance remains pretty much aware of their present surroundings, but they are also able to focus inwards more clearly and pull up the memories that are otherwise hard to get to. This is very much the same as police will use hypnotists to get more information from witnesses to crimes - under hypnosis they will remember much more detail than they can in their normal state.
The process was discovered accidentally when 'straight' psychiatrists used to hypnotise patients, trying to get them to uncover childhood traumas - which following Freud were believed to be the root of all psychological disturbances. The patient, under hypnosis, would be recalling some event that happened age three but then suddenly the patient would jump from being a small child to being an old blind woman walking through the forest looking for firewood or a soldier trapped in a fox hole or goodness knows what other scenario.
Various astute therapists began to wonder what was going on here as it was happening a lot. They began to hypothesise past life recall, naturally enough. Better than that, quite often the grown up dramas the patient was recalling were much more relevant to their present life issue, problem or phobia than anything from their childhood could explain adequately. Several pioneers started to do it on purpose and began getting very interesting results. This was around about the 1970s or so. It is a fairly new thing. Dr Helen Wambach has done a lot of work on verifying all sorts of interesting historical statistics and details with this approach and has built up a huge pile of data on the subject. Other practitioners just concentrate on treating their patients and don't care if it is 'really' past life memories or archetypal unconsciousness or fantasy or whatever as long as their patients find it therapeutic.
The whole process is not accepted by many mainstream practioners, as you can imagine, but new things are always resisted at first. Although, I have read stories about various patients who were referred to a past life regression therapist by a straight psychiatrist because their problem seemed intractable by more traditional methods - so there are some open minded people out there.
Past life therapy is found to be very therapeutic and much quicker than regular psychiatric counselling for many patients in many circumstances, depending on their issues. It is particularly good for phobias and stray fears and vague anxieties for instance. Even people with all sorts of physical symptoms (mystery pains, rashes, that sort of thing) have been helped. Once the person can pinpoint where it all began it seems easier to put it aside, lay it to rest, forgive, move on etc.
It is not harmful any more than regular counselling is - which is to say it is usually very therapeutic and good for you, depending on having a good practioner. It is wise to talk to a proper professional with proper qualifications who can provide counselling about the emotions which come up during the session, as opposed to someone who is just a hypnotist or a 'psychic' who will give you a 'reading' which might be interesting, but might only give half the story.