GOSH I wish I was prepared/organized enough to do this! :'D I consider it awesome if I can script an entire chapter at once (though I do have a pretty detailed note-form outline to work from)
I usually thumbnail between 2 and 10 pages ahead of wherever I am; it's a pretty focus-demanding part of my process, but I rarely have more than a couple of hours at a time to spare on it per week so it crawls ahead slowly. (I keep up with my update schedule by being fast with other parts of the process, haha).
All of my thumbnails are.... actually drawn pretty large. I have a small 4x6 notebook that I keep on me all the time, and I draw each thumbnail so that it takes up basically an entire page. Despite the size, they tend to stay pretty loose (I mostly do them that big because I also transcribe the dialogue into them so that I don't need a computer nearby when I sketch out the page itself later).
Here's a pic of one of my thumbnails, alongside of the full page it belongs to:
Since they're in a bound book and I draw them in order, organization kinda takes care of itself (same with the book itself). I keep all of my scans and finished (digitized) pages together in folders depending on what stage of the process they're at; one folder for layered PSDs with the text on its own layer, one folder for flattened pages, and one folder for the web-sized files.