Hi Webdev,
As far as I know, I do not think publishers have control/can manipulate which ads show up... the only thing you can do (which sounds like you're doing) is to optimize the page to a certain keyword.
Besides the basic SEO techniques (use keyword in title <h1> tag, have the keyword in beginning and end of the page, frequency of 3 to 7 times, have the keyword in a link on the page, etc), I have also read that it helps if you make the keyword <b> or <strong> ... to highlight the keyword.
Now, on a slight tangent, here is how you can pick keywords that would likely come with a nice earning per click:
This may be a review for most, but in case you haven't already done this (or if someone else does not know about this tool), go to your Google Adwords account, under Tools, click on "Keyword Tool"... if you don't have one, go to
https://adwords.google.com/select/KeywordToolExternal
1) enter a general keyword and click "get more keywords" (e.g. "cancer")
2) under "choose data to display", select "cost and ad positions estimates" from the drop-down menu
3) under "Calculate Estimates using Max CPC: US Dollars (USD $)" enter the maximum amount, which is $100
4) click on "estimated average CPC" and it will sort the keywords by the value of each keyword to be in the top spot (in our example, the keyword "prostate cancer treatment" has the highest bid value of $10.69)
Now, if you create a page and SEO for the keyword "prostate cancer treatment"... dependent on a variety of factors, the ads that show on your page will more than likely come with a nice earning per click.
Now, there are obvious flaws in this technique, because these keyword values are based on "Google Search" and not "Content Network", which means that's the amount an advertiser would have to pay per click for his/her ad to appear in the top position of the Google search pages. This advertiser may be bidding much lower on the same keyword in the "content network"... but I think it would be safe to assume that this keyword (even in the content network) will still generate a nice earning per click.
Sorry, this was long, but I thought I would share this with everyone, and hopefully it will help someone increase his/her earning from Adsense.