Do a little project. Go to google right now and type in "mature porn" (or pick a niche), now write down or bookmark the top 15 or 20 link sites that show up. (maybe repeat on yahoo and condense to top 15 or 20).
Now visit them and surf a bit. See a sponsor of yours underpromoted there? Good, now build a free site in that niche that meets the requirements..
Simple maybe. You already do it maybe. But filtering doesnt *start* at your index page.. it starts way before that, so use some basic tools and ideas to get a pre-qualified surfer to your site.
Also build your sites ads with the sponsor tour and members area in mind (request a members area pass, it can only help and can never hurt to ask). Use the info to accurately reflect what is actually there. Then the surfer see's the tour and see's you didnt bullshit them and promise them the moon. Gives some positive reinforcement in their mind.
Basically keep factoring out random luck as much as possible every step of the way.
I'd even suggest using campaign/banner tracking codes on your sponsor links wherever available. It's more data to use. If an ad spot gets you 90% of your total clicks, you could use that information right?
Anyway good luck!