When my mom and one of her good friends came to Thailand for my wedding reception we had a funny tipping incident. They had flown in from Bangkok the night before and took an early flight to Krabi, where I was already staying. When their boat arrived at Railey Beach
I ran down to meet them. The boatman who'd brought them from Ao Nang had helped with their luggage and my mom's friend pulled out 200 baht to tip him. I stopped her before she had a chance to walk back to the boat and suggested maybe 50 baht was sufficient, to which she replied "well, 200 baht is just over $6 and these guys are lugging our bags off the boat for us in the heat." It seemed right to her and back home it probably would have been.
I'm not a stingy person by any means when it comes to tipping. I know that people in Thailand don't make a lot of money compared to what we make, and I'd rather over tip than under tip any day. It's not that I didn't want our family friend to tip generously, but to tip generously within the context of local practices. Of course you can't do that if you don't know how much you're supposed to tip in the first place.
I've put together a quick guide to Tipping in Thailand that should answer most questions about how much to tip and when.
As for my mom's friend, she ended up giving the boatman 60 baht, which he was very happy with.