Recently I've been rereading Darkness over Tibet, and there is the following very interesting snippet:
In the opinion of the wise men of Tibet, the giving up of one's body for any purpose whatsoever to invisible entities so that they can temporarily take possession of it is one of the greatest crimes which a human being can commit against his soul. The Divine spark in man, his soul, is greatly offended and brutalized by them.
If the soul leaves such a being such a man remains soulless for all eternity and enters the downward current of life, which leads him down again in the opposite direction, through the whole process of evolution to nothingness and total annihilation. All the horrible types of animals and plants, they say, are incarnations of such falling entities.
The sungmis of Tibet (note: the ones that get possessed by demons as part of a ritual) must lead unnatural lives in order to retain the faculty to be obsessed by their gods or demons. They are not allowed to have sexual intercourse. (note: maybe in order to avoid 'cross contamination"?) They voluntarily abstain from many amenities of life so as to be able to go on with their business. They must live on a special diet. As to the food they take, I heard different stories, but it seems that eggs and various kinds of meat are strictly forbidden.