I almost always my favorite sketchbook and pen with me, at the ready to draw an idea for a new design, or perhaps a refinement of an existing design. It can be be something that my mind sees and I want to remember for later, or that I want to try to work out in two dimensions before moving on to actual 3D modeling.
As some of you know, I sketch using a Bic four-color pen. The four colors can represent just about any set of variables that my brain is juggling at the moment: exterior/lining, foam/webbing/fabric, zipper/seam/fold/edge. I'm not particularly methodical about which color represents what, just that there's a obvious difference in the specific drawing I'm working on.
Somehow I find that a square notebook (the EcoJot is my favorite) frees my mind up from strictly a horizontal vs. vertical duality. Sometimes I'll doodle something and find that by turning it 90 degrees it suddenly makes more sense, or becomes something so different that it sends me off in a whole new direction.
I'm not too fastidious about my sketch book, and often as not there are shopping and to-do lists side-by-side with phone numbers and quotations.
I have dozens of sketchbooks in my archives; maybe someday we'll scan and share a page or two from each.