Thanks! It took a lot of practice. Keeping them straight for myself isn’t so difficult–it’s like keeping characters straight. The more I’ve written, the easier it’s become.
What is more difficult is keeping it all straight for the readers. This can be tough. One of the challenges with fantasy is what we call the Learning Curve. It can be very daunting to pick up a book and find not only new characters, but an entirely new world, new physics, and a lot of new words and names.
I generally try to introduce this all at a gentle curve. In some books, like Warbreaker, starting with the magic system worked. But in Mistborn, I felt that it was complex enough–and the setting complex enough–that I needed to ease into the magic, and so I did it bit by bit, with Vin.
In all things, practice makes perfect. I have a whole pile of unpublished novels where I didn’t do nearly as good a job of this. Even still, I think I have much to learn. In the end of both Mistborn One and Warbreaker, I think I leave a little too much confusion about the capabilities of the magic.