One of the ways that I have had success in earning money from my blogs is via Google Adsense. This is a system by which, by signing up and adding a small amount of code to your blog, Google can place context sensitive advertisements wherever you wish them to appear.
By context sensitive, I mean that the Adsense system will read the text on your page and determine which ad would be most suitable. In this way more of your readers will find ads that interest them, and are more likely to click on one. Whenever someone clicks on an ad, you get paid!
I will write more about including Google Adsense in your blog shortly, but for now I want to mention a new feature that Google have just released. One of the sections of Adsense is the ability to earn from referring other users to the service, just as I am doing in this post. If someone joins the service by following a link from your site you get paid.
You can join Adsense and be able to refer others by clicking on the link in my Tools menu in the righthand sidebar.
I have been using Google Adsense Referrals for some time with some success. However, Adsense Referrals 2.0 is just being released to the wider public. It now incorporates the context sensitivity of Adsense Ads, and includes a much larger range of products to which you can refer your readers. It sounds like a winner to me.