Do remember the following things:
1) Always be respectful
Always be respectful towards the locals, monks or Buddha images.You can end up in jail if you try being disrespectful towards Buddha images as they are sacred in Thailand.
2) Dress in a proper way
I know, it's hot and humid. Especially, for women, you would rather walk with your shorts and tank tops. However, that is not a very good idea if you are planning to visit temples or sacred places. At the very least, try to cover your knees and shoulders.Remember, this simple act shows that you respect the culture of the country.
3) Do lower your body slightly
It is again having to do with being respectful. As you can see respect is a huge thing in most Asian countries.
4) Do take off your shoes
Don't forget to take off your shoes when you are entering someone's house or building which has a Buddha image within.
5) Try to learn some of the basic phrases at least
It won't be that hard to know how to say hello. Men say "Sawatdee-krap" and women say "Sawatdee-kah". Try to recognise the "wai" gesture.You as a foreigner are not expected to initiate the "wai" gesture, but if someone offers it first to you, try to return it back. However, never return a "wai" to a child, waiter, clerk, etc. Simply nod and smile in response.
The followings are not advisable in Thailand:
1) Never be disrespectful
Do not show any disrespect towards the locals, the royal family, monks, just never towards anybody.
2) Never try to touch the head
It is a very disrespectful act, even if you mean it as a joke. Head is sacred in Thailand.
3) Never cross your legs
Especially in front of elderly or monks do not cross your legs. It applies whether you are sitting on the floor or in a chair.
4) Never pass anything with your left hand.
Also, never point with your hand and never, never with one finger.
5) Try not to be so affectionate in Public
It is a taboo to touch the opposite sex in Public. Please refrain yourself from being too intimate or affectionate towards your partner in public.
6) Do not, I repeat, do not touch a woman publicly without her consent
As I have mentioned before, touching the opposite sex is still a taboo in Thailand. A lot of people in Thailand still believe that a woman can lose her face if she is being touched publicly.
7) Don't take Buddha images out of the country
Unless you get special permission, it is strictly against the law to bring Buddha images out of the country. Of course, you can easily buy them there, but try to avoid unnecessary problem.
However, these are only some general things that you need to be aware of when you are in Thailand. I hope these do's and don'ts can help you in some ways. Enjoy your stay in Thailand!