When I was doing a lot of posting to promote various sites. (up till 6 month ago) my four best converting "wife" sites where, in order:
Naughty At Home I don't know what her "secret" is but she seem to retain her members much longuer than any other.
Naughty Allie
Sammy 4 you
Amber At Home
Even after 6 months of, almost, no promoting of these sites I still gets new sales and rebill from my galleries.