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 standing there with his hand out waiting for a tip.

All public taxi rates are metered, and both Thais and local ex-pats commonly round off the fare upwards as a tip ( 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 acts as a tip of sorts, which is 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. i.e. 20 - 50 baht for the porter that carried your bags up to your room, or leave a small tip on your bed for the cleaning lady is also appreciated.

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

Masseurs, hairdressers etc also deserve a small tip 20 - 100 baht depending on the individual situation 

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 (they'll remember you too).

Of course, if the service is unacceptable or ordinary then don't tip. Residents in urban areas like Bangkok have grown accustomed to tipping. In fact, many a taxi driver has given their blessing and hope for eternal riches on the back of an unexpected tip. It doesn't hurt to give a little, and the smile of appreciation will light up your day.   

Etiquette

The King

The King is very highly regarded almost universally, 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).

At movie theatres before each performance the Thai Royal Anthem - Phleng Sansoen Phra Barami (เพลงสรรเสริญพระบารมี) or sometimes more simply as "Kha Wora" Listen (click link) You are expected to (YOU MUST) stand up during the playing of the Thai Royal Anthem.

Thailand has very strict  lèse majesté Law, and any act considered disrespectful of the King or Royal family WILL subject you to the pains and penalties of Thai Law.

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.

More tips on taxis can be found here; Bangkok Taxi Service

Temples and Monks

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

Always remove your shoes when entering temples (same rule if entering a persons 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.  Hence do not point to things with your feet, hold doors open with your feet, point your feet to the Buddha images, point at or touch peoples' heads,  or under any circumstance (especially women), touch  a monk.

Hygiene

One of the most basic etiquettes often overlooked by some travellers is personal hygiene, Thai people take their personal hygiene and appearance very seriously. As Thailand is a tropical country many synthetic fabrics from  can promote odor building bacteria, so showering often is essential and not just every couple of days. Local laundry options AWAY from your hotel are plentiful and cheap (40-50 baht per kilo). For this you can drop off your dirty clothes, and collect them the following day, clean, folded (perhaps even ironed) and packaged up like new. So there is no excuse for wearing dirty clothes.

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

Loud or abusive expressions of anger are also to be avoided at  all costs. Things in Thailand often don't go to plan, and visitors need to take a very chilled attitude to annoyances. Despite your annoyance, always remain calm, try to smile, 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 often best just to acknowlege a wai with a smile and a nod.

It is just not done to wai children, service persons etc even if they wai you first. You may though wai an elderly Thai you're introduced to, or even a monk, but the general rule is refrain.