Keep in mind when purchasing and/or extending your visa, you will need two passport photos of yourself, your passport (obviously), and of course, cash (don't expect to be able to pay with credit cards or check). It is a good idea also, as a matter of habit, to bring your airline ticket with you when purchasing your visa.

By the way, you will be required to purchase a round trip ticket to Thailand. This is because Thai immigration laws do not permit tourists to reside in Thailand. So, a one-way ticket would indicate you are not planning to leave (not such a bad idea!); so you're basically stuck buying a round trip ticket.

So what happens if you decide you love Thailand so much, you wish to stay forever? You cannot. Legally, you must leave the country when your visa and extensions have expired. Many foreigners who reside in Thailand use the expiration of their visa as a good excuse to take a trip to another country, such as Cambodia, the Philippines, or other fun places. However, there is a way to renew your visa without leaving the country. Some travel agents, for a fee of around $80, will hand carry your passport for you to Malaysia (or Laos) and have it stamped. I'm not sure how comfortable I would feel about leaving my passport in another person's hands, but a good friend who lives in Bangkok swears by it and has had no problems.