From what I have researched, retention rates for a membership site are tied to the level of service on the part of the site owner. Paying members who find high quality information and quick responses to their questions are those who are more likely to stick around for the long haul. Top-notch content and high levels of interaction are also what gives the perception of value. Some of the best ways to keep members coming back to a membership site include listening to their feedback and providing content that fits their needs. A lot of this carries over from basic customer service concepts.