Chris & Hamid,

Using similar ideas to take wind out of the picture, Ed Williams
(references below) derived the equations for an easy to fly method.  Hold
3 headings separated by 120-deg, measure GPS ground-speed -> true
airspeed:

 http://williams.best.vwh.net/avform.html#3gs

He also converts IAS to TAS even when Mach is not negligible (good for
finding calibration issues with OAT & airspeed):

 http://williams.best.vwh.net/avform.htm#Mach

I put both into a spreadsheet that works on computers and iPads:

 http://www.beechtalk.com/forums/viewtopic.php?f=3&t=49840&start=400

--
Craig