Michael R. Ruhm had an interesting article in 1989 in Crossroads: An Interdisciplinary Journal of Southeast Asian Studies entitled "The Lords of Ava: Spirit Rites in Northern Thailand." He wanted to examine the spirit sacrifices known as liang phi practiced by the Tai Yuan. He described three rites in which the leuk-lan, (children and grandchildren) offer food, drink and ritual goods of flower, incense, etc. to the spirits for blessings. During this time the medium may get possessed by the spirits.
He goes into the details regarding the rites for the Cao Mok Mung Muang (Lord of the City-Hiding Fog) who is one the tutelary spirits of Lampang. The rites known as phi meng are associated with the phi mot and phi ahak, who are ancestral spirits, which he clarifies as "spirits that have been handed down from the ancestors." The final spirit sacrifice is one held every four years for the five "Ancestral Lords" known as the phi pu-na cao-nai of the village of Ban Com Ping in Lampang province. Ruhm observed that all of the spirits are represented as supernatural princes. He concludes that the rites with their "unequal contests between human and spirit reaffirm not just gender, household structure, or princely power, but all these things and the entire social-cosmic complex of which they are a part."
It's a good lead for people who are interested in this sort of topic. He notes a number of interesting texts that may be worth tracking down. This includes the 1984 "Spirit Cults and the Position of Women in Northern Thailand by Paul t. Cohen and Gehan Wijeyewardene, and the 1984 article "Decline of Village Spirit Cults and Growth of Urban Spirit Mediumship." Both were in in Mankind 14 (4). They might be hard to track down but could prove interesting reading.