All you have to do is register with xbox.net and the offer is even open if you are already registered with them. This is for U.S. only.
Here are the one year subriptions that you can get: Computer Gaming, Xbox Nation, Electronic Gaming, PC Magazine, and Computer Shopper.
http://www.xbox.net/register.php
After you register, you can sign up for the magazines through the link below.
http://www.xbox.net/magazine.php