1) Immigration will need to see your original University Diploma. Yes, original. Yes, I'm serious. Really really. If you're told you don't need this, either they aren't getting you the right visa and you're not working fully legally or they've got strings to pull. This, is also not unusual but to get a one year teaching visa, you NEED your real diploma with you. Whether or not the policy is new, the enforcement has scaled up exponentially.
2) Female teachers must wear skirts in Thai schools. Internationals vary.
3) Do not be surprised when Immigration questions the legitimacy of your actual, legitimate passport. Also yes really. No advice on this one, just be prepared. If you ever deal with a police officer that claims a document of yours is fake, demand to go to the police station. It's possible the cop is fake, or just scamming. Asserting your rights to go to the station gets them off your back.
4) When applying for a Work Visa, never wear jeans to Immigration. When applying for a tourist extension, always wear jeans to Immigration. Or other touristy but not too obnoxious outfit. Don't wear sandals. Your approval or denial can hinge on how you are dressed.
5) What bank you use will be determined by the school where you work. There are no fuzzy lines blurring banking and education whatsoever, at all. *cough* Many banks will not give you an account without a work permit. This also varies by branch, bank, and mood of who you talk to.
6) Murphy's law requires that you will be seen by the same woman every trip to Immigration, and she hates your guts and thinks you're a teenage dirtbag even when you're in your mid 20s. That or she really just loves enforcing her rule book of logical procedures.
7) Visa runs are like changing the oil in your car. Completely normal procedure. If you start to get worried that going on a visa run is shady, you're in the wrong country. Oil change for your passport.
8) Don't even try to use PayPal abroad as an American, make sure your banking institution is not going to dump you because of the Patriot Act when you move abroad, and yes, you do have to file your US taxes both federal and state and maintain a US address (not P.O. Box) even if you don't have to pay any, you can't file electronically if you earn foreign wages, and you will find new reasons to hate the IRS you never dreamed of. Thank you, Patriot Act renewal and additions. They do give expats an automatic month extension to file taxes. Also realize any non-Thai cards incur a 150 baht ATM withdrawal fee for every transaction. Get a card that refunds fees if you cannot get a Thai card; or withdraw in maximum chunks to go awhile at a time. Also, many Thai banks will only withdraw from your savings account and not checking, because personal checking does not exist here. Be aware of this when choosing ATMs, it might take from your savings or simply say it cannot handle your foreign card.
9) Realize there is a very good chance you may be the only American at your work place. Some places are full of Americans, but I'm on my second job as the lone Yankee on staff. Be prepared for as much or more culture shock from this as from Thai culture.
10) Don't believe the myth that American automatically means better. My thieving, fake plane ticket issuing travel agent was US based. There is a travel agent here in Bangkok that never steers me wrong traveling in Asia, and I have found a more reliable US agent for flights back home. Do your homework.
And PLEASE, don't tie your shoes to the outside of your pack in a country where feet and shoes are reviled, and when all else fails, there is a large Leo beer and a Mai Pen Rai with your name on it.