Of all the Pay-per-Click programs, AdSense is probably the easiest way to monetise a website. Some blogging softwares (such as Blogspot and Wordpress) are even AdSense-ready i.e. they have widgets which you can use to place AdSense advertisements on your blog.
So.. you have decided to use AdSense. The question now is how to get the most from the ads you display on your site.
It's all about traffic, traffic and traffic. The more people visit your site, the higher are the chances they will click on your ads. So promote your site in whatever way you can think of. Social websites such as Twitters, Facebook and MySpace are excellent promotional tools. AdWords is also a good option but that involves some initial monetory investments from you. So you might want to think more on that.
2. Ad Placement
In general, the more visible the ad, the higher is the click rate. So putting a leaderboard banner on top of your site is really a good strategy. You can also put an ad in the middle of your content. While potentially it can perform well, it can also be annoying to your readers. The fact is, ad placement is pretty subjective and the way the ads behave is different for each site.
3. Ad Size
It's common sense that your ads should fit the layout of your site. Choosing ads that push your content out of the screen edge is really a bad move. You might also want to mix and match ads of different sizes and place them strategically on your site.
4. Ad Colours
You may think that choosing ad colours that contrast your site is better since it attracts people's attention. While this is not entirely wrong (remember, the behaviour of ads is different for each site), it is actually better to have ad colours that blend with your site. But this is also a subjective matter. You may want to experiment with different colours and see how it affects your click rate. You may also want to remove the borders from your ads since, in general, borderless ads perform better than ads with borders.
For advice or support on AdSense optimisation, you may want to join the Official AdSense Google Group or visit the AdSense Support pages (an account is needed). There are also a number of forums out there which you can Google and join.