(1) Ignore touts who accost you to try to persuade you to take a car they can arrange for you.  They may tell you it is a taxi or limousine or minibus.  Ignore them!  You will be ripped off and may be taken to a different hotel.  Deal only with the Public Taxi attendant at the queue or with attendants behind the AOT Limousine counter.  

(2) Do not take a stray taxi!  A stray taxi is one which happens to be passing by and catches your eye.  Ignore their offers and inducements.  You will be ripped off and / or taken to some other place.  Typically, once you have got into their taxi, they will switch on the turbo-meter which will tick over much more quickly.  In the end, you will find you are being charged two, three, four or more times the normal meter fare!  

(3) Spend a few minutes before you leave for Bangkok to surf the web to learn about the latest scams and shenanigans perpetrated by taxi and tuk-tuk drivers, and for any recent changes in fares and tariffs.  There is nothing to be said for naivety and selective ignorance in these parts.