DO understand the meaning of the "wai." Wai is when a person puts their hands together close to their body at chest level and gives a slight bow. The higher the hands, the more respectful the wai. It can mean "Hello", "I'm sorry", or "Good-bye."
DON'T point with one finger. It's considered impolite.
DON'T show affection in public or even touch someone of the opposite sex.
DON'T pass something over someone's head. The head is considered sacred in Thailand.
DON'T point with your feet or use your feet to touch something. Feet are considered dirty, because they are the lowest part of the body.
DON'T talk with your hands or put your hands in your pockets while talking to someone.
DON'T step on a threshold when going through a doorway. Step over it instead. Thais believe that a spirit lives in the threshold.