The book Democracy, Development and Decentralization in Provincial Thailand by Daniel Arghiros is about the local elections, taking an anonymous district of Ayutthaya as the example. Even though the actual book is most comfortable to read, you can read the part I am referring to in Google Books as well. In fact, it was a search result from Google Books which made me notice and finally buy this book. Even a book nearly completely readable in Google Books can serve as an advert to buy it. And hopefully for out-of-print books it'll have something like gBooks in the near future as well. But back to the topic...
Already with the thesaphiban reforms at the begin of the 20th century, the posts of the subdistrict headman (kamnan, กำนัน) and the village headman (phu yai ban, ผู้ใหญ่บ้าน) became elected posts. However in the district observed by Arghiros it was usual till the 1960s that the one elected was in fact chosen by the district officer. And also it was usual that the post was semi-hereditary, when the former headman died or retired often his son was elected as his successor.
In the time of about 1960-1990 the candidate elected was usually the one who had the largest kin-group. Also, before the 1970s the post of a headman was actually not really popular - a lot of work, not much power, and only a low salary from the government. This changed when the Tambon councils were given funds for development in their area, and with the increased possibilities also the temptation to redirect some of that money into their own pockets. Another big change happened at during the 1980s, when the district changed from being predominately agricultural to industrialization. Now the entrepreneurs targeted these offices to secure their power and access to funding, and also the vote-buying became rampant.
Further changes came in the 1990s. Till then once elected the office holder stayed till retirement age without any need for reelection. Now they have to run for reelection every five years. Also most of the tasks and powers have since been transferred to the local administrative units, Tambon Administrative Organization (TAO) or the municipalities. But that's the topic of the later chapters I still have to read.