Almost all of the ideas I have stem from problems in my life and work. Ideas I have on a topic accumulate slowly, and eventually reach critical mass. At that point, I think I have enough to make a coherent post. I might suggest first getting into the habit of incubating your ideas in some system, whether a set of text files under version control like I have or Evernote or whatever works for you. You'll make a lot of connections this way that you would otherwise not make. (Unfortunately I have almost no time to turn my semi-organized outlines into posts at this point, but that's life.)

Some of my other ideas are responses to other things I've read. For books, writing a book review or summary (especially if it includes a bit more than is in the book, i.e., other things you are aware of) might be a good approach for this. Replying directly to things you've read online also is pretty easy.