As someone has said before, server logs are gold mines (of course for a site that has already started rolling out not one that has just been incepted).
1- Check the traffic logs. 2- Pick (highly) visited pages according to your logs (preferably those that have been visited via by a direct link from a search engine). 3- Edit those pages to increase and enhance their content. Make sure to tweak them to better target the search query phrase that brought the visitor to them in order to achieve higher position in SERPs for that particular page for such a search query.
* If the search query did not hit an internal page head on, or was slightly (or strongly) diverting from the content of the page it points to, then create a new page around this search query and add it to your site. That's how you grow your content. This last method is what has been suggested before many times, and has been strongly suggested (and by the person who mentioned that server logs are gold mines). I also add that using an traffic analyzer (other than the server logs) can be also convenient.
Just thought to share my thoughts and 'recent' experience.
As for new sites, sure this method will not work. This is only for sites that already have some content and get at least some traffic from search engines.
I have always had better results leaving good pages alone and exploring and enhancing subtopics that are not getting traffic.
Look at your server logs and see what people are not searching for on your site, match that up with what people are searching for on the search engines and you will see big holes in your content. Modify the pages that are performing poorly or add content that people in your subject area are looking for but not finding on your site. These actions will reap bigger benefits than optimization of already performing pages.