I'm a whiteboard fanatic. Not only do I have two whiteboards in my writing cave where I scribble all my super-secret notes, but I've also started wallpapering my walls with post-its.
(Yes, they're all different colors... and yes, I buy them in bulk.)
PSA: Don't turn on the ceiling fan when you have post-its for wallpaper.
When I started writing Beneath the Fallen City, I wasn't thinking about turning it into a series. It just sort of happened. The story ended up being something bigger than what could be contained within one book.
It's required some... creativity. Especially with staying organized.
I use a series bible to keep track of my characters (and their elemental alignment):
Yep. That's my digital cheat sheet. Don't zoom in -- SPOILER ALERT!
You peeked, didn't you?
That's okay. Over the span of six books, I've managed to collect quite a few characters. As I told my editor, in my most recent book, I went a little "Game of Thrones". Sometimes, killing off a few people is the best way to keep the character count down.
Well, that and it's a little therapeutic.