Do not disrespect the King or any of the royalties.

The monarchy in Thailand is very highly regarded. In fact it's illegal to criticise the royalties and it's a crime that carries a very long prison sentence. You may have read about the 57 year old Swiss national who was sentenced to 10 years in prison for spray painting the Kings Bhumibol Adulyadej picture.

Do not use drugs in Thailand

The penalties for drug related crimes in Thailand are severe, stay far away from it, even a small amount of Cannabis could give you years in prison while a more severe offense such as trafficking could give a death sentence. Do not use, do not carry and do not stay around someone else using drugs.

When visiting temples

When visiting a temple or other religious places you should be wearing proper clothes. preferably long pants, long skirt or shorts that are long enough to cover your knees. Shoes needs to cover your heels.

Don't show affection in public

Openly showing affection is frowned upon and is considered something that should be kept in private. Although this is generally tolerated all around Phuket and in other tourist area's, it's not part of Thai culture.

Dress properly

Do not wear revealing clothes in public, Thais are generally more conservative then westerners, please respect their culture by wearing your shirt on when you leave the beach and ladies, keep the top on at the beach.

Try to stay calm at all times

Be patient and don't show any anger no matter what happens. Anger is considered a weakness and smile will get you much further no matter how bad the situation may seem.