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.
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.