Sketchbooks. I tore blanks out of various sources and worked on them in pieces, and then put them back together at a later date. I carried many of these small "books" with me during my extended travels to asia (Taiwan & Hong Kong).
It's important to have a way to throw out ideas without judgement. A sketchbook shouldn't have to be refined (but it can be, depending on your creative energy at the moment); it's a place where you're free to release unfiltered ideas. Regardless of whether you're working on a "personal" piece or commercial work, exploration is a vital step in making something real. It's almost impossible to encounter "happy accidents" in a sterile digital environment; the chaotic, quick nature of the sketchbook can help promote that.
