I'm a wine professional, living in Bangkok and travelling frequently throughout South East Asia. I'm a huge fan of Laos and Cambodia and off the beaten track travel. Feel free to contact me at: www.facebook.com/mil.elephants
While this might sound obvious, it can be very important to be aware of cultural differences that might cause distress or offence in a foreign land. Thailand is no different. Despite the friendly relaxed atmosphere that most visitors experience while in Thailand, certain acts or statements can cause extreme offence and even get you in a lot of hot water.
Do not discuss the Royal Family in any way shape or form. Even if you are inclined to say something positive there is the possibility that it could be misconstrued in some way. Any confusion over what you say can be taken the wrong way, and as their is not presumption of innocence in Thailand, you're quite likely to be going to jail for a very long time.
2. Feet. Showing the soles of the feet to people is seen as an offensive act and should be avoided. This is difficult if you're on the beach and enjoy the sun, the soles of your feet will naturally show. I've found myself caught out by this while sitting in a cafe or bar and crossing my legs to get comfortable and have accidentally been showing my feet. I've also seen Thai people start fights with foreigners over this, keep them on the ground if you can.
3. Throwing something to someone can be seen as offensive. It's regarded as being aggressive and and act normally associated with an angry teacher throwing something at a student. If you say to someone "hey, can I throw you this", that's perfectly ok, but often it will make them feel uncomfortable and it's best to be avoided.
4. Arguments and raised voices. Typically anger is not shown publicly in Thailand. There are exceptions, but shows of anger at poor service, at a difficult taxi driver, or a government official will get you absolutely nowhere and will no doubt make things worse. Generally Thai people keep their emotions to themselves and any extreme emotion, be it happy or sad is not viewed as suitable for public viewing.
5. Don't bargain too hard when shopping. In most part prices outside of purely tourist areas are not heavily marked up. Discounts can be had for bulk purchases but most of the time you're getting close to the best price available. Also avoid trying to bargain in shopping malls and mainstream retail. It's no different to the west and prices won't move just behave you're in Asia.
Now after your all down about the things you shouldn't do, here are the things you should (must) do.
Do get away from Thai main tourist areas of Bangkok, Phuket and Pattaya. Thailand is a wonderfully diverse country, full of varied countryside and wonderfully, helpful and friendly people. In my opinion your best experience of Thai culture is going to be out side of major tourist destinations.
2. Learn a little Thai. It's amazing how positively Thai people will respond to you when you make the effort to learn a few words. Just simple please, thank you and excuse me can make a huge difference. Numbers are especially useful when shopping. Here is a very hand guide including pronunciation.
3. Get spicy, get out in the streets and try some street food. A lot of it is spicy and may be challenging if this isn't to your taste, but give it a go. There are also many great Thai desserts and snacks to keep you happy. Bangkok is the street food capital of the world. Try the bugs, their great with cold beer, just like potato chips.
4. Take in a festival. The big one is Songkran, in the middle of April, but there are many to choose from, and many will lead you off the beaten path to a faraway town or city. Festivals usually involve a lot of noise, music and action, mixed liberally with Thai Whiskey and Beer.
5. Have a real Thai massage. Traditional Thai massage lasts for 80 minutes, so if you only do 30 minutes or an hour it's just not enough. Go for two hours, it's cheap and invigorating. Listen to your bones creak and crack back into place as they twist you in ways you thought impossible. It's my Friday night ritual, getting ready for the weekend.