When you advertise on blogs
You need to know How to write killer BlogsAds. Brian Clark wrote BlogAds campaigns for Audi, Levis and Sharp TV. I know I clicked through on all three. I haven't purchased anything from the three companies, but they did get my attention, which is a solid start.
Update 2012 June 03: This originally linked to a post on IndieWire with an article from Brian Clark including five tips for creating a great BlogAds campaign. That article has since vanished. The next best thing is a post from the same timeframe that offers blog advertising tips from Clickz. It's many years old at this point, but the advice is still relevant.