I think a good idea might be to send the traffic to a nice thumbnail gallery like page, where there are sample pictures and perhaps sample movies for the pay site.
Just as an example, here's the gallery I did for BigNaturals yesterday:
http://www2.cleanadulthost.com/amate...fany/index.htm
(quite basic I know, but it took like 5 minutes)
Go to the page, pretend you're a surfer...and then watch the sample movie clip. You're either reaching for your cock after that, or your credit card. Perhaps both, but you get the idea.
Now, if the person clicks through to the pay sponsor, it's pretty much out of your hands and you won't know if they signed up or not.
If, on the other hand, they close your page without clicking through to the pay sponsor, then you might try a fast loading text console with the word "FREE" very visible somewhere on the page advertising your free sponsors.
There is a way to do this in javascript, if the person is closing the window, hit them with an exit popup...but if they are clicking on a link of yours, no popup.
Hope this helps