Personality and exclusive content. Lot's of both.
In the begining we just shot pictures and video with little interaction, now with all our sites we try to bring more interaction between us and the models. That is how
www.onetwistedpair.com developed, we brought more of ourselves into the content, otherwise, it is just another porn site. We are also working on making more of a community, things like contests where members and affilates can come hang-out at our shoots, etc.
Having stuff members cannot get anywhere else helps as well. Also, string out the updates, when we add a new girls, it takes at least a few weeks before all of her content is up. We also are impletementing a list of future updates, so they can see what is comming up.
It doesn't hurt to add in lots of video plug-in either, it is just more to keep them busy.