No matter how you intend to spend your time in Thailand unless your a Thai citizen your going to need a visa. (In some cases a Thai citizen needs a visa if they carry another passport). For people who want to live permanently in Thailand you'll need to take special care with your Visa application. There is a saying amongst expats living in Thailand. "Everyone has problems in Thailand, but none more so than the dreaded Visa Run".
Getting the proper visa is essential to living in Thailand, and if you don't get the right visa it will cost you money, time, and aggravation. Tourist Visa's can only give you up to 6months after that you'll need some sort of visa that gives you status of permanent residence.
Visa Waiver in Thailand
The visa waiver also known as the Visa on Arrival grants you stay in the country for up to 30 days. After that you'll need to cross the border and come in. If you walk over the border you get 15 days, if you fly you get 30.
Retirement Visa in Thailand
Retirement Visas are for those over the age of 50 and who have no criminal record. You'll need to get a health check which is easy to do if your in thailand, i've even heard of doctors giving you one without even looking at you, it's not uncommon. You'll also need about 800,000 baht in a thai bank 3 months before you apply, that money has to stay at that level the entire time you intend to stay here. you can't touch it! Otherwise you can get proof that you earn 65,000 baht a month.
Education Student Visa in Thailand
Most people who intend to stay in thailand permanently start off with an education visa. the Education visa is a great and easy way to stay in Thailand. If you intend to live here learning the language can be very benificial and you can make some great friends too. Courses typically cost around the 25,000 baht mark. If you apply while your in Thailand you'll have to go to Malaysia and come back, but after that you'll never have to leave thailand again and can renew your studies for a further 9 years, by that time you should be able to apply for the retirement visa!
If you decide to do the ED Visa for up to 9 years, every 3 years you must leave the country and return and do a test with the Ministry of education.
A quirk with the ED Visa is this, if you live in Chiang mai or Phuket you only have to pay 1900 baht a year to renew your visa. However if you live in Bangkok or Pattaya you have to pay and renew your visa every 3 months. Makes for living in Chaing Mai or Pattaya a lot cheaper.
Work Visa In Thailand
The work visa is another great way to live in thailand, more often than not you'll be a teacher wanting to teach english in Thailand. This visa is difficult to get for the company not so much for you. Most places who have hired foriegn workers already know what to do. You'll still need to leave the country every year.
The business Visa is a good visa to have but you must leave the country every 3 months which gets annoying and you'll have to go back to your country every year. It's a good star though and the cheapest visa you can get that let's you stay in the country for a year.