Check out GrayWolf’s AdSense tips section. While you’re at it, be sure to read his AdSense: Why Bloggers Don’t Get It post. Allow me to quote two of the article’s strongest points:

Now before you get all fired up about it, understand that I don’t have a problem with you having a personal blog or sharing it with the public. However your expectation that it has value outside of your family/friends/community, is a serious misconception.

and

So what exactly is a commercially viable blog? Don’t think of it as publishing a blog, think of it as publishing an online magazine.