I played for free for 3 months using this method.
Steps:
-Go to a drugstore (ie. CVS) and buy "Pre-paid" credit card, I used AMEX.
-The next day come back and buy another one using the card you bought the day before
-You will now have two cards, one should be empty the other one full.
-Login to your WoW account and update your subscription.
-Enter the already used credit card information and click enter to start your subscription. (Depending on the origional value on the card the subscription time will change. If you are using a card with only $25 on it, you can't buy a 3 month subsciption.)
-You now have a "dead" card and one with $25 on it still. You can then buy anything you want with your Living card (including another one to repeat the process) and use the spent card to re-new your subscription for another month.
You will need to do this for each month but you will never need to pay again
Let me know if it works for you.