Using your website’s stats software (be it Google Analytics or AWStats as an example) its 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...