Soul Retrieval has been a powerful technique for thousands of years. Native tribes basically use it in an emergency, e.g. somebody goes into a coma. A child almost dies and they'll have a tent full of children wailing for him/her to come back, inviting the soul to return to the body. As a technique used so "civilized" people can be more present, in order to live a more integrated life as people, it has only recently been well known.
I started researching the subject in earnest around six years ago. I had names such as "fragments", or "aspects", a term taken from the work of Jane Roberts. The methods I first developed were less effective due to the models I was using: mental concepts such as time and space, position in relation to the body, why did you split off. Those all return mind or attention units, but are not reliable in returning your ability to feel. A lost soul part believes it is the person, so one must ask it only questions that would invite integration, rather than questions that would evaluate separation.
Two years ago I apprenticed with the Soul Retrieval methods of Sandra Ingerman, who teaches with the anthropologist/shaman Michael Harner. The method she teaches involves lying on the floor next to the client, you journey in an altered state of consciousness brought about by monotonous drumming, and you find the soul part according to its own reality as to where it is, and with permission you bring it back into "ordinary" or consensus reality, and blow it back into the client's body. You briefly describe what you saw and the emotion and age that the lost soul felt it was when lost. Then the client spends perhaps 3-4 weeks integrating. This can be an emotionally dramatic phase, as memories of the trauma that caused the soul part to leave tend to return with the soul part, and this can take a period of weeks where supportive friends are needed.
A lack of childhood memory is a sure sign a soul part is gone. This is very common on this planet, as non-tribal societies tend to be out of touch to the feelings of the developing child, especially in the name of education. The soul part returns and the memories return with it. There can be quite some emotional pain during the integration period, as some definite ability to feel returns with each soul part. Things the client couldn't feel before have now come to the surface.
Soul parts are found in poetic rather than physical spaces, which cannot be reliably found by mechanistic means. Clearing techniques used to the end of time will never get to these as they are too complicated. During Sandra's training it became simple to track with and find a soul, even if it were in an improbable sounding space, such as hiding in a cake in a cave inside a mountain in Fairyland.
I have a huge respect for Sandra's technique and acknowledge her as the person who has made soul retrieval available so that any person who is as fragmented as you might need to be to arrive on this planet can be more present as a person.
My training with Sandra included some exercises that were a whole lot of fun. Some involved Sandra and Michael beating drums for 20 minutes, and half the students hide in non-ordinary or shamanic reality, e.g. in a bowl in a box in a cave at the end of a rainbow, and the other half have to track and find them.
In the process of a few days of exercises we learnt to track and find soul parts and deal with this whole thing gently. Soul parts are handled by very gentle invitation and negotiation. You could snatch a soul part back to consensus reality and abruptly blow it into somebody's body, but it won't stay.
Monotonous drumming as used by many 'primitive' tribes facilitates the shift of consciousness, and once one starts feeling the way rather than thinking, it is actually rather nice. With a male and a female drumming together, you can often start shifting consciousness within 20 seconds. No one needs drugs to learn to shift consciousness when there are two people drumming.
The bottom line question to the client is "How would your life have to change for you to be present and honor the qualities of this part of yourself?" If a person lost a major part of himself at the age of 3 months, and wanted to continue to look at life in the same old way, and not change any habits, that part won't return. Babies look at things very differently and newly, and most adults have lost that. Two years before the "Child Within" became such a popular concept for New Age workshops, I knew it would be popular, and thought I'd be able to get in on the act, but I just couldn't spot what I didn't know about it. This was that the child almost always isn't within, it's normally out, lost in space somewhere. Quite often after a Soul Retrieval, a person now can make sense of those workshops. "Now that I'm here, I can use this data."
Some people have wanted a more "scientific" sounding term than "Soul Retrieval". You could effectively use an alternate poetic term, but no mechanistic term will work for anything more than getting bits of mind or energy back. These return of their own accord after effective Soul retrieval. You could alternatively waste years finding attention units you have left behind in various universes. If YOU are here, you attract what is yours like a magnet.