1) Use text ads, not image ads.
Statistically, image ads dont get as many clicks as text ads.
2) The ads should be close to your content.
People are reading your content, so their eyes will have a greater chance to see a keyword that prompts them to click.
3) Dont rely on standard ad placement, or ad sizes.
Many people have trained themselves to ignore 468×60 banners placed in the traditional spots. Experiment with untraditional placement, and ad size. Possibly even the bottom of your content.
4) Use multiple ads.
It is reasonable to place multiple ads per page, as long as you dont dilute your content.
5) Consider white borders.
Classic ads online have lines boxing them off from the other content on the page. However, you might test a white border which for a white page would effectively render the border invisible. No border means no block to stop the eye.
6) Match borders to your site colors.
Another option: use a border color that matches your sites palette as closely as possible. The ads look more at home and often get better clicks. My blog, Shimon Sandler, has a great example of this strategy.
7) Create content to match most expensive AdSense clicks.
You can check which keywords are generating higher costs per click, and write your content accordingly.
8) Create ads that look like your Navigation.
The Engadget (www.engadget.com) blog has a great example of this.