I just called up Xbox Support because my LIVE is renewing tomorrow and I dont have enough money on my card at the moment. Anyways, lets get to the point:

Step One:
Call 0800 587 1102 (this is for UK, i'm not too sure what the number for US is) and then press 2 for Xbox Live, then press 1 to Cancel Membership or to turn off Auto-Renewal. All I wanted to do was turn off Auto-Renewal.

Step Two:
Once you get to speak to someone, you must give them your Email and they will chase up your account. Once they've found your account they'll ask you for the last four digits on your card, the Email once again, the name of the person who owns the account and who they are currently speaking to and the full address.

Step Three:
They'll now ask what you need them to do (turn off Auto-Renewal) and why you want to do it. I just said I was going to buy a pre-paid card from the shop. The person I was speaking to then offered me a 12 month membership for something like 23.99, DO NOT accept this offer.

Step Four:
He then told me about a website called rewards.xbox.com, where you get rewarded MicroSoft Points for renewing your memberships, filling out quick monthly surveys, once again trying to convince you to stay on with Auto-Renewal.

Step Five:
Next I was offered three months for the price of one month, DO NOT accept that either.

Step Six:
Finally, he offered me a free month to decide whether I wanted to renew for a year once more. I'm not sure if you'll be able to do this month after month, i'll try to find out next month.