Clustering is defined by Gabriele Rico in her "Writing the Natural Way," a book I highly recommend. On her website, Rico describes the clustering process:

"A non-linear brainstorming process, clustering makes the Design mind's interior, invisible associations visible on a page. Clustering becomes a self-organizing process as words and phrases are spilled onto the page around a center. The Sign mind (Rico's term for the right brain) begins to see pattern and meaning, and the writing flows naturally into a vignette."

Rico uses "Sign mind" for the "left brain" and "Design mind" for the right brain.

Dustin Wax describes the process a bit differently:

"Here's the basic idea:

1. Write a word in the middle of a sheet of paper.

2. Circle it.

3. Write down the first word or phrase that comes to mind and circle it.

4. Draw a line connecting the second circle to the first.

5. Repeat. As you write and circle new words and phrases, draw lines back to the last word, the central word, or other words that seem connected. Don't worry about how they're connected — the goal is to let your right-brain do its thing, which is to see patterns; later, the left-brain will take over and put the nature of those relationships into words.

6. When you've filled the page, or just feel like you've done enough (a sign of what Rico calls a "felt-shift"), go back through what you've written down. OK. Erase whatever doesn't seem like it fits, and go on to number the circles to impose a priority on them. By working together, your right brain and left brain can produce what amounts to an outline. At this point, you can either transfer your numbered clusters to a proper outline or simply begin writing in the order you've numbered the clusters.

By the time you've started reviewing your clusters, your brain has done much of the work of fleshing out your ideas; all that remains is to put these relationships into words, which is what your left-brain mind excels at."

Rico's clustering technique gives you access to all that is in the various conscious and unconscious or subconscious parts of your mind on a subject. When you're done—and you get that thing mentioned above as a "felt-shift," or a "kerchunk," after 15-20 minutes, to signal that you're done—-you will have a sort of map of what you know about a subject. The process not only lists what you know -- it gives you an actionable research list of things you will have to look up.

I cluster everything—-my plan for the day; a phone call; a book (apply the process to the table of contents, then to the individual chapters); a business plan; a talk.... Clustering empowers you -- and gives you a wonderful feeling, too.

If you're stuck, trying to get your book out, start by clustering your table of contents. Then cluster each of the chapters.