The Nando Times offers an ad-free subscription version of their site for $1.00 per week. But they also do something interesting by saying, "you only pay for the weeks you use. Just set up an account with your credit card and you'll have access to the site for the next seven days. When you come back next week, or a month from now, you can buy another week of access with single button click." While this approach eliminates the promising possibility (from the service provider's point of view) of charging people indefinitely, whether they use your service or not, because they never get around to cancelling, it could increase subscriptions those who are wary of starting yet another auto-billed service that they may not use and won't get around to cancelling.