how do you process payments?
stympy: I'll plug myself here... :)  If you are using Rails, take a look at http://railskits.com/saas/ -- a great starting point for your Rails apps to not only do the recurring billing (with Auth.net, Braintree, etc.) but also do account management, upgrades/downgrades, etc.It has a lot of the work done for you, so you just plug it in and go.