Well...to tell you the truth...check around the trading forum and you'll probably find someone selling off a 12mo subscription card.
I netted one last week for $25 off the boards.
I was just about done with a 2mo trial (been using them for the past 8mo), and called up Xbox live support and asked how we could go about it as I didn't want to keep the gamertag from my current trial. I was able to apply my sub card, and they did a tweak so the next time I logged in to Live, it'd prompt me to change my gamertag. Work'd out great.
Best of luck.