Well man. I really think that you should work on your site. Building it will make more of your daily traffic come back to you. That way you'll need less new sources.
About advertising the index page... If the free site's here:
site.com/free_sites/teen_babes/index.html
and you put 2 galleries with teens and shit... instead of ads pointing to the sponsor you could advertise the site.com's index OR you can combine those - sponsor links and site links. You may have big success with sending traffic from galleries/sites to the index page, BUT you should always think about what the surfers are expecting to see. If I was you, I would make a better rating system and would add complete paysite reviews, just to enrich the site's content and make it a little bit more meaningful to browse the inside pages.
I'm giving you suggestions for enriching your site ONLY because I get 10 times less traffic on a single site and I don't ask "how can I get traffic" questions. 70k daily is a very good number, it should be used wisely and instead of increasing it, convert it. If you're doing good, do better
-- Andrew