Advice for negotiating as a tourist in Thailand

Learn your numbers and use them any time you are talking with people, even if you don't use any other Thai.  People seem more likely to accept a price when you tell it to them in Thai.  We were negotiating with a cab driver and as soon as I said the price in Thai, he agreed to our price.  This is not bullet-proof evidence, but I think it's pretty good.  I get that sense from interacting with other vendors (w/ English numbers and w/ Thai numbers) also.

Speaking of cab drivers.  I think we made a mistake by saying where we wanted to go and then asking how much.  The meter will always be cheaper than negotiated price b/c the drivers will quote ridiculously high numbers to start out.  One time, we just got into a cab, said where we wanted to go, then the cab driver turned on the meter and it was 73 baht.  Previous trips of around the same distance had been 150 - 200 baht!  Also, don't get in the cab and close the door until you get the driver to agree to what you want.  Once the cab is moving, you lose your advantage.  Seems simple, but when you're not really planning these things out ahead of time, it's easy to just automatically climb in and shut the door.

This is not bullet-proof evidence, but I think it's pretty good.