To be fair, taxi drivers taking advantages on tourists is not unique to Bangkok. However, the incapability of taxi drivers to understand English may increase your chance to wander aimlessly in Bangkok in a cab. Don't expect they will at least know the English street names. They don't.
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At Least Do This: Get a Thai name of your destination and present it to the driver.
Glimpse the map to get an idea what direction and how far is your destination. If the taxi goes to a different direction for some time, probably the driver misunderstood your destination or he is not taking the direct route. Don't feel bothered to reconfirm your destination if this happens.
Know where some major landmarks are (e.g. famous hotels) so you will still know where you are after the cab has traveled many streets.