Duncan over at Blog Herald has a good guide to Promoting your blog with advertising where he looks at Google’s Adwords, the BlogAds system, smaller advertising firms and direct advertising on other blogs.
I’ve used Adwords twice and like the idea of it but had pretty average results. Apart from that I’ve never advertised any of my blogs. Having said this I’m interested to hear others experiences. Do you advertise? When, how, what were the results?