one HUGE thing that has helped our retention is regular updates and [Imore importantly informing your members when they are getting updates. After logging in, our members see a landing page that says something like:
April 18 - added 2 more videos - click here
April 15 - new live show added, check it out here
April 12 - new photos added, view them here
April 11 - click here to see the 3 new videos added
etc...

that way when a new member joins they can see that this is a dynamic site that is worth sticking around and also all existing members always see what is new right after they log in and can get to them by clicking directly on the link provided. And yes, you do have to update your sites at least once a week to keep members coming back. Plus give them a variety of updates, like Lee said, sometimes its nice to check out some pro photos or videos too!

One other thing, have you thought about dropping your trials? I know this seems strange, but when we dropped our trials our sales only dropped slightly but more importantly our retention skyrocketed. The overall effect was a few less members but more cash for both us and our affiliates.

good luck with it, let us know how it goes.

cheers,
Luke