Some Do's and Dont's
In general Thais are pretty forgiving of visitors who make the odd faux-pas but if you know a few of the basics maybe you can avoid them.
His Majesty The King of Thailand is revered and respected as no other monarch in the world. Considering his 60 years on the throne in which He has dedicated His life to His people, it's not surprising. It is in fact the law not to desecrate his image or speak badly of him. For 
Thais this law is not so much a legal matter as one of inner belief. As a visitor you should respect that, especially when you are handling money bearing the Kings picture. In the West you may throw money in disgust if you are forced to pay for a service which was delivered badly. If you do that in Thailand it could very quickly land you in a whole heap of trouble!
Gan Hai or Tam Bun are integral to Thai culture. Literally they mean giving. So often you will encounter the desire to give or please and maybe not understand why. It is a very deep part of the way Thais live and go about their world. From a Western perspective that may seem odd; for someone to want to give for no return! And sometimes forcing a reciprocal "gift" can be offensive.
From this comes the desire by Thai people to avoid conflict. If a visitor looses their temper, however justified it may seem to do so, in the eyes of Thais, you are not a very nice person. It's best to be very patient, count to 1,000 if need be and then try to deal with differences in a calm voice and apply gentle reasoning. If you blow up you have lost for sure!
When you enter a temple, restaurant and even a persons house, the chances are, if it looks Thai, you should remove your shoes. You'll probably see other shoes outside so that can be a guide but to be on the safe side, leave them at the door.
And while on the subject of feet, try not to show the soles of your feet, or shoes, to people when you sit. Pointing your feet in this way is regarded as very rude, much akin to pointing at someone in the West.
