One of my editors once told me that sometimes, to get to the really good stuff, you just have to put a lot of your lesser ideas down on paper. To me that's the importance of sketchbooks and notebooks -that is, making a habit of scribbling and putting your thoughts down. Most of those thoughts and ideas won't really go anywhere. But the point is to keep doing it.
I've kept sketchbooks for years, but it's only been a few months since I added a smaller notebook into the mix (the little, black Moleskine), and it's been a boon. I carry it almost everywhere and I use it to sketch rough comics, sneakily draw people in coffee shops, write thoughts, journal entries, and generally scheme. Here are some pages (I've blurred out a couple silly lists):
You know, I've loved the idea of the battered, illustrated journal/diary ever since seeing Dr. Jones Sr's grail diary in Indiana Jones and the Last Crusade. How many of us still keep pen and paper diaries, I wonder?