Ok, I'll go first. These things seem to help keep eyeballs on my site, some of which then go on to the ads.
1. Content. I offer evergreen content not found anywhere else (that is, until the pages get scraped) and have 'regulars' as well as SE referrals that come back everyday to see what's new.
2. Good logical layout (like Google, Yahoo!, Amazon). I try to make my site as easy to use as any of the major portals, even though my site is only a tiny fraction in size.
3. Syndication. I use RSS, Twitter and recently added Facebook.