As a Happy New Year present to all here...
After a lot of fiddling, I've a few hopefully useful pieces of information regarding getting the most out of AdSense.
You'll get the most out if you make your ads relevant to the content of the pages. The more relevant, the higher the clickthrough will be, and the better advertisers will be since you're feeding them better traffic.
Here's how, in my experience at least, to make the AdSense robot's life as easy as you can.
Use decent HTML
The AdSense robot appears to like well structured but clear HTML. Use style-sheets to separate layout from content, and try to keep your HTML page (the only thing AdSense looks at) as simple and clear as possible. For example, many people will put FONT tags around headlines on a page - don't, and instead surround a headline with an H1 tag, taking care to re-define an H1 tag in your CSS. This appears to let the AdSense robot understand what your page is all about.
Make these clear but relevant to your page. If you run a widget company called ABC Inc and have made a page about red widgets, don't put 'ABC Inc' as a title - put, at least, 'ABC Inc - widget maker - red widgets' there. The title appears to really help AdSense.
Similarly, the page name is useful. For the above page, don't call it abc.inc/article.asp?id=238. Instead, try and get your content management system to cope with abc.inc/widgets/red/ - even queries appear to have a good effect, so abc.inc/article.asp?id=238&title=red_widgets might get a slightly better response.
Let the AdSense robot in
If you run a site where a portion needs login before the content is visible, then the AdSense robot needs to see the content too. Rewrite your security slightly to allow everything with IP addresses 220.127.116.11 - 18.104.22.168 in without any problem.
Using an SID? Stop it!
Refer to pages consistently
page.asp?id=12&type=writing works exactly the same as page.asp?type=writing&id=12 - but to AdSense, they're probably two separate pages. Write all links to pages like this consistently, so that AdSense doesn't think they're two separate pages.
Placements and colours
Ads that fit into the look and feel of the website and in navigation blocks - like in a header - don't appear to work as well as ads that fit in the main content. This user, at least, uses two separate ads - one in the header, and one (most of the time) directly below the main content.
Get everything right, and your Google AdSense ads can become useful content for your visitors. If they're useful content, they click on them, and you make money. And if they're relevant to the content of the page, advertisers will thank you because they're highly targeted leads.
It's been a lot of playing about, but I hope this article is useful for everyone here.