Hi Rei-
The works of Edith Fiori on entity depossession give as good a generic introduction as I've been able to find. However, there's seems to be a bit more to it.
Samuel Sagan has written an interesting interpretation called
Entity Possession in which he classifies four levels of intrusions. The lowest is mindless and grossly material, the next somewhat instinctive with a little life force, the next is deliberately adaptive, but without a sense of self, and the highest level of Sagan's hierarchy has a well-devloped ego structure and personality. The system Sagan presents is logical and clear.
In my practice I've encountered two main types of hitch hikers, the obvious everyday spirit with personality, and the other with utter undirected panic that overwhelms all personality. On one or two instances I've found a aethereal being that had stuck onto a person, claiming to have been misdirected by other spirits, plus a very rare third instance of bundles of spiritual energy more or less maladaptively stuck onto muscle and organ groups.
For the general case, such as family members etc, it's a matter of ego and pride, confusion and projected expectations, plus hatred and rejection and the fear that accompanies it. The "black hatred" that we occasionally encounter comes out of the level of anxiety and fearfulness that these guys feel. I've had hitchiking entities who were afraid to go into the Light because they were certain that their tormenters awaited them there. I've had people killed by animals who felt that they must now act like an animal. And I've had a few spooks who thought that they were demons - because it had been so long since they stopped being people, and they had adopted ways of parasitic existence.
The treatment is essentially educational. First, it is necessary that everyone be aware that these are dead people. Death is OK, because it is never fatal. (When a spirit laughs, then we're OK.) If necessary, we review life, how death happened, unresolved desires and fractured dreams, and also what is the purpose of being a "stuck-on spook" as compared to going into the Light to learn a better way to accomplish all those good things etc.
Then we have to develop the sense of clarity a little. The entity has extremely imited abilities. Attachment to the everyday world is only through those very few traits that overlap between the spirit world and the material world. These especially include the extremely basic strong emotions. The entity has no proper memory, so the causes of events must be perpetually present in order to think about them. That means that if Uncle Louie is attached to Willie's hatred of rutabagas, then that will be an obsessive thought about which the attachment will revolve. Usually this turns into a few moments of expressing emotions, as well as realization that there it is really not possible to "help" or "cause" the host to perform in any specific manner. The entity's awareness is thus turned to the global situation and away from the specifics of attachment.
The third part is reintegration, abandonment of fear and hatred and acceptance of infinite lve and forgiveness. This can be scary, because we're all pretty imperfect. But the story of the Prodigal Son has served me the same way as a sniper's rifle, tergeting and eliminating fear of judgement. In a few cases I've had to point out that the well-meaning fire-and-brimstone preachers who threaten hell and damnation were simply wrong. There's neither hell nor is there any kind of rejection from God. The worst we get is a feeling of embarrassment that we didn;t do a better job - but we get that wth the ability to go back and do it right.
I work in a chatty friendly manner, as these are they guys that I've dedicated my life to serve. There are no souls so needy as those wh fancy themselves damned and dominic. What a pity! Often I ask the entity if there are friends in the Light from their past lives - and there always are. Then I ask their friends to tell the entity whether or not the Light is a universally better place to be. I also often point out that they can return from the Light -and get the friends to verify this. In that way, they are not losing anything, but they are gaining abilities.
Once in a while someone doesn't know how to get into the Light, or feels inadequate to go there because of past issues. Then the friends can reach down and give them a hand up. I've had some pretty rickety souls perched on a cloudy outcropping in fear of falling back into chaos, but once they get there for a moment, it is possible to point out how much easier it is to accept that in spite of all the ways we screw up, it still is OK, and God is accepting of all of it.
I have a protocol for basic past life work on my web site
mbs-hypnoclinic.com as well as a written booklet at
Lulu.com and if you want a complete text I have a text "Introduction to Hypnotic Regression" and another book of related readings "Developmental Psychopathology", available at
BookSurge.com or
Amazon.com under my name "David P. Armentrout, PhD". Please note that these are simply complements to Bruce's books. If you've read Bruce's books then you've already gotten the same ideas, and it's simply a matter of application. And, in all honesty, Bruce is a more competent and more readable author. Oh well.

I hope this helps-
dave