If you want real retention you have to look at why products that do retain, do so well.
People buy Ford cars over and over again because they are good value, they buy Rolls Royces because they are superior products. In the porn world no one can match the retention of buyers like Playboy, Hustler, Barely Legal, Evil Angel, Private in the old days, Vivid, Anabolic etc. These made a great product that customers bought month after month. The magazines had 1/3 to 1/4 of their sales on yearly subscriptions.
One common factor in them all was they paid the best money to keep buyers buying.
The adult Internet decided content was to be bought as cheap as possible and as much money as possible was to be spent on finding new customers to replace the ones leaving. Worked bloody well for a few years. Welcome to 2011.
The customers won't stay if you want to give them a cheap product. That rules applies if you're selling porn, Jumbo Jets or Baked Beans. New customers don't replace old ones because the new customers, as far as porn is concerned, are on Tubes. They know from the crap on those that few sites have little better to offer.
Maybe adding live scenes might break the cycle. Adding more of the same poor to medium of what the site already has won't. We have been on that cycle for years.