Chris Lema is back to blogging, and a recent post of his covers how you can get free users to subscribe to paid memberships.
The entire post is a good read for anyone trying to make it with subscriptions. Some highlights: (1) Determine how many different segments of users you have. (2) Understand those segments, and map out their lifecycle demands. (3) Create compelling offers and remember that when it comes to converting from free to paid accounts, people will pay for goal acceleration.