Gnu has a very interesting project for changing your mac address every time you want. I have found it very useful - apparently for security reasons!
The project has the name: macchanger and i will not tell you how to installed it !
I will tell you how i am using it. Become root and type: crontab -e
add the below line
@reboot macchanger -r wlan0
Now, every time you boot up your machine the mac address on you wireless card will be a random new one !