If you have the Turkcell Gold Packet you are eligible to get the Havas airport transfer bus for free.
How: Send a text message to 2222 - type message in capitals ÜCRETSiZ HAVAS (S with a squiggle) shortly before your journey.
You will receive by return a pin number which you simply show to the driver when he comes to collect the fare. He will make a note of your phone number and the pin number and you will be richer by 20TL.
It's that simple...
This offer is available in Dalaman, Ankara,Izmir and Antalya - it is not available in Istanbul.