I'm surprised this is a mystery. It's similar to what makes you come back to a restaurant:
1) Is the food good?
I go back to restaurants where I like the food. I buy trials to others' adult sites all the time, just to see what they are doing, and so far, I've found only one to which I have allowed rebilling to occur (
http://www.ultrawired.com). But before you sign up for a partnership program with them...they don't have one.
2) Is the product's price position appropriate?
If a product is a good value for the dollar, people will keep coming back for more. This applies to adult sites as well as restaurants.
3) Is the service good?
Can you talk to the maitre'd or the chef? If you have a complaint, do they respond promptly and satisfactorily? Every complaint is an opportunity. I used to work in direct marketing, and one of the little-known facts of direct marketing is that the most loyal customers are the ones who complained...and their complaint was resolved swiftly and satisfactorily. The more amazing and pleasing your response, the greater the loyalty.
4) This applies in particular to membership sites (not restaurants): The site must be updated regularly. I used to be lazy and updated my own site (
http://www.girlsdotcom.com) once a week. When another webmaster clued me in that it's better to divide the material up into chunks and put it up ever 2 or 3 days, my retention went up quite a bit. I have gotten a trial on some sites, liked the material, but after a few days went by and nothing new appeared, I decided it must be running on auto-pilot and cancelled out of my trial.