I keep a few different sketchbooks and am pretty loyal to this practice. Each of them have a specific purpose. My "Eye-Book" sketchbook is perhaps my most important, as it records both images and words. It teaches me how to become objective about my own work and keeps me tapped into my raw feelings.
I also have a bedside moleskin sketchbook. I take it with me everywhere I go. This book is a bit more free for ideas or for anything creative in general: characters, funny animals, to-do lists, notes, etc.
I keep two other "sketchbooks," which are not on regular upkeep. Sometimes I'll just use loose paper or scraps, like post-its, envelopes. or cards. And then there's my faithful Journal Project that's ongoing and forever.