DON'T export any Buddha images, works of art or antiques. DON'T initiate a 'wai', only reply to one. Do not 'wai' servants and children. A 'Wai' is the traditional Thai greeting of placing the hands together. DON'T drink the tap water.
Bottled water is available everywhere.
DON'T carry anything through customs for anyone else unless you know the contents. Penalties for drug trafficking are severe. DO follow common sense health precautions and check with your local doctor on current vaccination recommendations for travelling within Thailand. DO take care of your valuables at all times and report any loss immediately to the nearest tourist police office. DON'T buy gemstones or jewellery unless it is from a reputable dealer.