Thai social customs are generally forgiving of visitors who make an honest effort, but a few points come up often enough to be worth knowing in advance.
The wai
The pressed-palms greeting is used as hello, thank you and goodbye. Visitors aren't expected to initiate it, but returning one when offered is polite.
The monarchy
Thailand's monarchy is held in very high regard, and Thailand's lèse-majesté laws make disrespectful comment a serious legal matter — a topic best avoided entirely rather than joked about.
Feet and heads
Feet are considered the lowest, least respectful part of the body — avoid pointing them at people or Buddha images, and don't step over someone's outstretched legs. Heads are the opposite — avoid touching someone's head, including children's.
Public displays of affection and temper
Overt public affection between couples is generally more muted than in many Western countries, and visibly losing your temper or raising your voice in public — even over a genuine grievance — is considered a loss of face for everyone involved rather than a normal way to resolve a dispute. Staying calm gets better outcomes in almost every practical dispute, from a taxi fare disagreement to a hotel booking issue.
Beachwear
Swimwear is fine on the beach itself but generally not worn into shops, restaurants or temples further inland.
Photographing people
Asking before photographing individuals, particularly monks, sea-gypsy communities and market vendors going about their day, is good practice — a smile and a gesture toward your camera is usually enough to get a clear yes or no.