Are there any special instructions for making SEO for sub-pages? We own a website which has 2,600,000 pages indexed by google. Each page has its own different targeted keyword. We are planning to get traffic from these sub-pages, traffic to index is not important. Until now index page gets around 30k UV daily, but sub-pages does not drive traffic. We have a lot of related backlinks, but all of them goes to index. Sub-pages are video pages. Keyword is video name. Website is 15 months old. We reached such number of indexed pages, but until now we cannot monetarize it..
Treat them as home page or individual page where you build unique links and special keywords. Best thing you can do for your on-page is, make a very good internal linking structure to let PR and ranking juice pass between your pages.
I have an almost similar site, with lot of content sub pages. Because I use WP, I use the easy wp seo plugin. Basically on page stuff like keyword density, h tags and so on, followed by building atleast a few 100 backlinks to each of these pages. Besides the above, interlinking one page to another also helps.