A step I rarely see advised but that's helped me a lot is to think about it from an intuitive, common sense level. Imagine you're one of your typical readers in today's economy. (Find out who your market demographic is, if you don't know it already.)
Would your ads, as they're presented on the page, be juicy to that reader, in these particular subjects as they're accessing your content?
Also a low CTR might mean irrelevant ads, which might come from lack of targeted traffic.