At Restaurants: About $1 per diner for the waiter.

At Hotels: About $1–$2 per bag for the porter; no tip necessary for the housekeeper or the concierge (service charges are included at hotels of two stars or above).

Guides and Drivers: About $1 for taxis; $2 per hour for private drivers; $10–$20 per person per day for tour guides (who also tip tour drivers, so don't worry about that).

Who Else?: If you ever find yourself at a local masseuse, a three-dollar tip at the end of the massage is about right.

Dollars Accepted?: Yes, preferably two-dollar bills (see Cambodia).

P.S. A common feature in Thailand is the ubiquitous bathroom attendant. Some of them might even throw a towel over a man's shoulders while he's at the urinal. Fifty cents, or about 20 baht, should do it there. It's also common to get a hot towel and drink upon checking into a nice hotel, but no tip is necessary, as the service is included.

Source : CNTraveler.com