Using your website's stats software (be it Google Analytics or AWStats as an example) it's fairly easy to see which of your pages are the most popular. When you write a page you never really know if a specific page is going to become popular in the search engines or not. But statistical analysis does give you the insight you need to find out what your most popular pages within a website are. Once you see a page is bringing in a little bit of traffic you should revisit it and see if there are any matching affiliate programs available that you could promote. You already know what people are searching for to find the page via search engines, so if you can offer them a suitable product it is a win:win situation. You earn some extra income and your visitors find a) valuable content and matching products.

Another good way of matching content to affiliate products is to talk to your competition. Pretend to be interested in advertising (if they know who you are, pretend to be somebody different). Ask what their most popular page is and what the matching keywords are that people use to find your competitors webpage. Now you know what your next article should be about and what keywords to target eventually. Of course there is a little more to it, but think about it - you are just duplicating your findings from before where older content is used to identify a new opportunity to make money from affiliate marketing. When you write the new article for your own website - make sure to have unique content and not to duplicate your competitor's content.

Once you have opportunities identified the question arises how to monetize it. Let's assume you have identified a webpage and an affiliate product/affiliate program. You can either just link to the affiliate product with your affiliate link (make sure to cloak the link as Google does not like affiliate links) or you update the webpage with some product related information (example: short review). Make sure to do A:B testing to see which option turns out to be better. A thorough test will help you to identify the best way to re-monetize your popular content the best way.