I believe RPM can be increased via keywording. The more fine-tuned an article is in regard to keywords, the more specific the ads are in relationship to what an article is about.
If you write an article about cowboy boots, and you really haven't put much work, if any, in choosing keywords (use the Google Keyword tool) for your article, the ads being displayed usually will have nothing to do with cowboy boots, thus the reader has little or no interest in the ads.
People are more apt to click an ad if its relative to why they came to your article in the first place---BECAUSE THEY WERE INTERESTED.
Additionally, the higher a keyword pays, the higher the per-click-rate, which, of course, the higher the revenue.
And, I'm no expert, I have lots of articles which still need to be properly keyworded. I have lots of reworking still to do.
Lastly, my biggest holders in RPM are not even articles, they're actually just simple bookmarks, correctly keyworded of course. You don't have to pay to learn about RPM or any other part of SEO. Google will teach you for free. they have endless volumes of info in the help section of Adsense.