Another option to consider is what the music site rdio.com does. They offer a three-day free trial and then ask for a credit card (which they won't charge) to

Another option to consider is what the music site rdio.com does. They offer a three-day free trial and then ask for a credit card (which they won't charge) to extend the trial for a longer period. This way you get more people to start using your site and have a way to quickly get a credit card number.