As I've mentioned, I'm working on a massive series and I found myself creating characters, then jotting down information about them, only to lose that information.
So I created a wiki to keep track of everything. It sounds really complicated, but it really isn't. As I get closer to publishing, I'll share the Wiki link with you. Suffice it to say, my wiki is my secondary brain when it comes to keeping track of hundreds of characters, many locations, and a plot that spans several years.
I do something similar for every book I write. I keep a little book notebook about the book and I split it into sections: Who, Where, Notes, Timeline, Details. I cut out pictures of houses to use in the book, floor plans, pictures of people, and jot down anything I want to in this book. I have one for every mystery I wrote, and it's come in so handy -- it's a secondary brain for each book where I can turn to find out my details.
Someday I may part with some of those old notebooks, but for now, I still enjoy browsing through them. It's like falling into the past and remembering the angst of writing and the fun I had when the book released.
My wiki is very like that, only it's digital, not paper. And believe me, it's come in handy to make sure nothing falls through the cracks!
J L Wilson