i signed up for my new gt last night per josh's request. when i did it, i got a month free trial. i think i figured out how to keep getting free month trials. it's worked twice so far.

sign up for a new live.com email (same as hotmail, just live.com at the end).
use it to get a new gamer tag on your xbox (not xbox.com)
say no to a subscription
after that i've gotten the offer for free month trial
opt to put in payment info later
account is automatically downgraded when the month is up

don't know if there's a limit or anything, but it has worked twice for me...