Always read the sign-up pages carefully. Some sites will include pre-checked boxes for other sites that you have to un-check or you're signing up for them as well.
If you don't realize you signed-up for the other site(s) and unsubscribe from it, they will usually automatically bill after a few days.
This site reviews porn sites thoroughly with warnings about these pre-checked pages, who they bill with, how much they charge, what kind of videos/pics are available along with amounts of such, how often they update and whether or not they use DRM or allow downloads. http://www.gaydemon.com/reviews/
There's also no need to pay more for a "one time charge" subscription unless you're just too lazy to unsubscribe before the month is up.
If you decide to sign-up, be sure to go through JUB front page (porn galleries) so they will get the credit.
NEVER "throw" your Visa number in on a website that you don't fully trust and one that you can't file a lawsuit against.
Do you really want to go to court and hear the announcement "Next on the docket, the case of Jamie versus Busted Straight Boys"?
No one goes to court for $35.
Visa/Mastercard allow you to dispute a charge and they will put it on hold while they review it. You don't pay credit cards in advance so it should get resolved.
Never use debit cards online. If anything goes fraudulent, your money is gone immediately and it could take months for the bank to put it back while they investigate. For that matter, don't use debit cards at restaurants either. Especially where the waiter/waitress takes the card out of your site for a while.