Kipp Bodnar gets it right in his blog "10 Simple Strategies for Business Blog Content". I found this to be right on not only for business but for whatever you are blogging on. This is a very good reference and should be part of your book.
I am not going to re-list all of the items however I would like to bring out a quote from the blog.
"The reality is that the web is full of too many great ideas and different types of content, that you should never run out of things to blog about, regardless of your industry."
This is so right. Read and use Kipp's list as both an example and a guide. Print it out and put it up on the wall, when you run out of ideas read it again.
10 Simple Strategies for Business Blog Content: