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.
It has a lot of the work done for you, so you just plug it in and go.