Google AdSense ads are contextual text-based ads that depend very much on the content of the page they are displayed on.
Obviously AdSense looks at several different thing to place the page at its correct location in the taxonomy so relevant ads can be selected and shown.
And consequently there isn't just one thing that can be changed to get AdSense to pick different ads.
One could argue that because it's such a complex algorithm that determines the theme of the page it doesn't really make sense to talk about keyword research and optimization in the usual sense.
There are basically two different reasons why you would want to do keyword research and change your pages.
1. To get rid of irrelevant ads, so you don't waste ad space on ads that almost certainly get 0 clicks because they are far away from what the visitor search for and expect to find.
2. To show higher paid ads, so you earn more per click.
So is Google AdSense keyword research something that should be treated differently from regular keyword optimization work?
Yes it should.
A page serving AdSense ads must serve a dual purpose.
1. It must attract enough search engine visitors by focusing on keywords with high demand but little competition. This is the way keywords are normally selected when you want the pages to generate traffic without having to put all your resources into tweaking and tuning the page.
2. It must attract high paying AdSense ads to maximize the revenue generated.
Let's say you have a page about "keyword optimization".
Through regular keyword research you have decided that "keyword optimization" should be your primary key-phrase.
Google show a favorably low number (4,760) of competing pages for "keyword optimization"
But a quick look at http://uv.bidtool.overture.com/d/search/tools/bidtool/ reveals that the maximum bid for "keyword optimization" is only $0.76
I know that there isn't a 100% correlation between bids for Overture keywords and for Google AdWords, but it's good enough to demonstrate why you can't rely on ordinary or traditional keyword research when it comes to maximizing Google AdSense income.
So what we are after now are additional keywords that are related so they can we woven into the content of the page - and help force AdSense to pick higher paying ads to show on your page.
"search engine ranking" would be such a phrase.
Google knows of 193,000 pages competing for the phrase "search engine ranking", so it's highly unlikely that you should be able to rank high for that phrase.
So traditional keyword research wisdom would say that it shouldn't be targeted, but when it comes to AdSense you have to think differently about this second keyword.
A quick look into the Overture View Bids tool show us that the maximum bid is $4.21
So the price an advertiser is willing to pay for clicks to the "search engine ranking" keyphrase is five and a half times more than the maximum bids for "keyword optimization".
So besides targeting "keyword optimization" to get targeted traffic to your page, you should also target "search engine ranking" to increase the likelihood that Google's AdSense will pick a higher paying advertizer.
An obvious tool to use would be the Site BuildIt Keyword Manager! if you have already decided to go with the benefits you will get from owning an SBI-based site.
WordTracker (of course) would also be an invaluable aid and you can purchase this service for as short as a one-day period.
Ken Evoy also did a rather lengthy additional Google AdSense review which should come in handy.
For an invaluable tool for generating supporting keywords you must take a look at Theme Master ,which does a great job of picking supporting keywords that helps define the theme of your page.