Tipping in Thailand in generally common practice, although not to the extent or the amount that is practised in many western countries. All public taxi rates are metered, and both Thais and local expats commonly round off the fare upwards as a tip (DO NOT GET IN THE TAXI IF THEY WON'T PUT THE METER ON!). If the taxi fare on the meter is 90 baht give about a 10 baht tip, or if you have been stuck in traffic 20 baht would be a nice tip.
High-end tourist oriented restaurants and major hotels may include a ten percent service charge in the bill. For hotels in busy tourist areas, it is customary to tip hotel staff. Upcountry hotel staff may not expect tips, but of course they always appreciate a few baht in appreciation of their efforts. For example, it is customary to tip the bellboy 10 or 20 baht for carrying a load of heavy baggage up to your room, and this is practised by Thai guests even at the smaller upcountry hotels and resorts.
In all restaurants except for soup shops and roadside food stalls, Thai people will leave a tip of coins left over from paying a bill, unless a restaurant is more upscale with professional wait staff, and then a cash tip - perhaps less than 10% - is usually offered.
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. Of course, if the service is unacceptable or food not good (WHICH DOES HAPPEN) then don't tip. Residents in urban areas like Bangkok have grown accustomed to tipping. In fact, many taxi drivers have given their blessing and hope for eternal riches on the back of an unexpected tip. It doesn't hurt to give a little.