Go to your analytics and look for your organic keyword referrals (in Google Analytics, the route is Traffic Sources -> Sources -> Search -> Organic). You can either simply scan all the terms for nice, valid long-tail keywords to convert into content, or you can set the time frame to something fairly long (try a 3-, 6-, or 12-month span depending on your traffic flow) so you have a lot of data to look at, then check for trends. You could, for example, look for query keywords (terms that start with “what,” “why,” and so on).