This infographic appeared in my inbox. I am not sure whether all of this would work for every kind of blog and every kind of audience -- for example, it stipulates that the article should take 7 minutes to read, and that therefore it should be 1600 words long. That is about three times longer than most other blogging experts say.
Still, it has a few interesting suggestions. As will all of these kind of things, I think the thing is to take from it anything that seems useful and applicable to you, and to try things out.
