Firstly before I start, let me point out that Thais are some of the happiest, most hospitable people I have ever met. Nearly everyone in Bangkok greets with a prayer and a 'Sawastecaaa' as a means of demonstrating their deepest happiness for the fact you chose to visit their country.
Taxi drivers on the other hand can be a menace and on some occasions will do everything they can to bully unsuspecting tourists into handing over large wads of cash or running up the meter for very short journeys. In all fairness, this is not a generalisation about all of Bangkok's taxi drivers, just the majority of them who feel that not treating tourists properly in the interests of securing a fast buck will not go unnoticed or forgotten as I aim to demonstate in this article.
It always helps to have a piece of paper with your destination written in Thai - this helps to reduce the chance of a misunderstanding between you and the driver. If you do not have a piece of paper, ask a local taxi rank official to tell the driver where you want to go and ask them to tell the driver to use their meter. Show the paper to your driver when hailing him on the street or at a shopping mall or hotel. Some drivers might refuse it on the basis of it being a poor fare, although illegal, what can you do about thie except hail the next taxi that comes along.
During daylight hours, never allow a taxi driver to start driving without starting the meter. One driver drove me for nearly two blocks whilst claiming that he could not read the paper I presented to him to show my destination in Thai even though a taxi rank official at the shopping mall I left had told the driver verbally where I wanted to go - taxi drivers will do this in order to drive a fair way only to then claim that the meter should have been started earlier and to try to barter an unmetered fare - always refuse to accept this and get out of the taxi at the first opportunity.
When asking a driver to start the meter, some will grumble about it, others will start swearing about it - do not fall for this. The idea is they want you to feel guilty about forcing them to use the meter, do not let them. Another driver began swearing profusly about the Thai police being corrupt and using all matter of profanities i.e. 'F*cking this... f*cking that'. Just sit back and let them vent, and under no circumstances engage in any debate about the police, military or the King. It is illegal to insult Thailand's King and you might find that insulting the police and or military will land you in hot water because of who those institutions report to.
At nightime, especially at weekends, be prepared to barter after 12pm - The Skytrain/BTS stops working at 12pm and so taxi drivers will happily take advantage of this by refusing to use thier meter and trying to barter their fares. Out of all of the days I stayed in Bangkok, only one driver used the meter after hours, the rest were unscrupilous. If you have to barter, bear in mind that most 4km journeys in Bangkok will cost around THB60, therefore, bartering up to THB80 is probably acceptable give or take a few kilometers to your destination but do not accept the usual starting offier of THB100 or even higher in some cases.
Help your fellow travellers by reporting rip off taxi drivers to your hotel - Some hotels will present you with card stating the licence plate number of your driver at the point of entering the taxi, keep these and tell the hotel about any offending drivers when you return to the hotel. The hotel might bar the drivers from picking up passengers at the hotel again or might report the driver to the police.