Hello! As I wrote in the topic! I have been doing this almost since I started playing WoW, so in my case I have been playing about 150-200 absolutly free.
What I do is that I pay 1 month with a special service that my bank provides, I don't know how many other banks uses this but it's called an e-card on my bank.
This is an e-card (taken from the website): e-card from Swedbank is the safe, effective and easy way for cardholders to shop online, without ever transmitting their actual debit card details over the Internet. By using an e-card, the cardholder can eliminate the risks of card fraud and identity theft that result in card numbers being exposed and potentially compromised.
Using Swedbank's online banking website, the e-card service means that the cardholder can generate unique numbers linked to the cardholder's actual card account. These e-cards are complete with expiry date and signature panel code (***2/CVC2), and can be used in place of real debit card details while shopping online.
So, I create a new not physical card every month to pay with with only the amount of money needed to pay for 1 month of playtime, and when the time is out my account always says "PENDING" or "FROZEN" but I can still log in to it for 3-5 days, I guess this is because it's trying to grab money from my card.
This will probably work with a normal credit card but you won't have any money on it when your gametime expires, but I'm not sure how many times you can re-activate a single credit card.
It would be pretty hard to confirm this but if you can use it, try it out! :=)