Tipping 

Tipping is NOT customary in Thailand, there is absolutely NO mandatory requirement to tip anyone, but small gratuities for great service are very much appreciated. Unlike some other parts of the world, you will never see a Thai service provider with his hand out waiting for a tip.

All public taxi rates are metered, and both Thais and local ex-pats commonly round up the fare (i.e. 51 baht fare rounded up to 60 baht).  Most restaurants and hotels include a ten percent service charge in the bill. This surcharge already serves as a tip of sorts. It's combined and shared among all employees at the end of the month in addition to their meagre monthly salary.

Tipping in hotels is not expected, but again is always appreciated, for instance 20 - 50 baht for the porter that carried your bags up to your room, or 20 baht left under your pillow for the cleaner.

In all restaurants it is customary to leave behind any loose change in coins as a tip. In more upscale restaurants, with professional wait staff who provide excellent service, a larger tip of 5%-10% is quite common. 

Massage ladies commonly get a more substantial tip of 100 baht because they get only a pittance out of what you pay the house, and often wait around all day for a customer.

Bear in mind that the majority of workers in the hospitality and service industries in Thailand earn very little, so a small tip goes a long way and will generally bring out extra enthusiastic service.

Of course, if the service is unacceptable or merely ordinary, you're free not to tip at all.

Etiquette

The King

The king is very highly regarded in Thailand, as evidenced by the pictures displayed everywhere.  Do not say or do anything disrespectful of the king or the royal family, even to the extent of stomping on a Thai coin or banknote which has been dropped and is rolling/blowing away. (It bears an image of the King's head, and is highly insulting to be touched by your feet.)

Before each performance at movie theatres, the Thai Royal Anthem - known as Phleng Sansoen Phra Barami (เพลงสรรเสริญพระบารมี) or sometimes more simply as "Kha Wora" is played. (Listen here) It is mandatory to stand up during the playing of this Thai royal anthem.

Thailand has a very strict lèse majesté law, and any act considered disrespectful of the king or royal family could subject you to severe penalties.

Bangkok Taxis

When hailing taxis, it is common to keep your hand horizontal, fingers facing down.  Holding your hand with fingers up is considered rude.

Temples and Monks

When visiting temples, dress conservatively. Women particularly should wear long skirts or trousers and cover their shoulders and knees. Many temples state as you enter that photography is not permitted. Even if there is no sign, please be respectful and consider whether it is appropriate to be taking flash photographs in a place of worship. Even more so if monks are present worshipping!

Always remove your shoes when entering temples (the same rule applies when entering a person's home), and do not sit with your feet towards the Buddha. Sit either cross-legged, or with your feet tucked behind you.  In many Asian cultures the feet are considered the lowest, dirtiest part of the body, and the head the highest. So do not point to things with your feet, hold doors open with your feet, point your feet to the Buddha images, or point at or touch peoples' heads. Under no circumstances should a woman ever touch a monk.

Hygiene

One of the most important points of etiquette often overlooked by some travellers is basic personal hygiene, Thai people take their personal hygiene and appearance very seriously. Since Thailand is a hot. humid tropical country, showering frequently is essential and not just every couple of days. Local laundry shops away from your hotel are plentiful and cheap (40-50 baht per kilo). You can drop off your dirty clothes and collect them the following day, clean, folded (often beautifully ironed) and packaged up like new. Basic clothing like t-shirts and shorts are cheap at markets and discount supermarkets. So there is no excuse for wearing dirty or worn-out clothing.

Thai people are very modest in nature, and public nudity is frowned upon. There are NO nude beaches in Thailand.

 "Cool Heart" 

Loud or abusive expressions of anger and temper trantrums should be avoided at all costs. Many things in Thailand often don't go according to plan, and visitors are advised to take a very calm attitude to various nuisances. Try to remain calm and smile despite your annoyance, and in the end you will probably get what you want. 

The Wai

Unless you know the right way to 'wai' (sounds like "why"), the appropriate hand position and to whom, it is usually best just to acknowlege a wai with a smile and a nod.

Do not wai children, service persons etc. even if they wai you first. You may wai an elderly Thai you're introduced to, or even a monk, but the general rule is to refrain from wai-ing anyone.