Originally Posted by NickW
If you scoot to the departures level on arrival, you can usually pick up a cab that's just dropped off a passenger, and they won't charge you the airport pickup surcharge.
I wouldn't recommend this to a first-timer though, as you don't have anybody available to translate your destination, and no course if things go wrong.
I use the upstairs guys if I have to arrive during the late evening rush and there's a humongous queue downstairs, but the rest of the time the 50 baht (c'mon, it's $1!) is a small price to pay for the convenience.