I'm sure lots of people know about this, but it's news to me. I let my 1 year + month expire and a thing popped up on the dashboard offering me 2 months for 2 bucks.
 
Score. Of course you have to remember to turn off auto-renew.

Better advice... don't buy an Xbox if you don't already have one.  get a PS3 where you never know if you have online play.  It's so exciting not knowing when the network will shit out on you :P