I started carrying around this little goat notepad:
Aside from having a good picture on the cover there, it's tiny. Smaller than a moleskin. Which is great, because I'm constantly losing those awkwardly-sized, expensive little black books.
Anyhow, little notepads are great for doodles.
Or for jotting down some storyboards just long enough to remember. Be sure to write it down more detailed somewhere else--this picture here doesn't make much sense otherwise. Or maybe this is how I really see things.
When in doubt, draw merlin. take advantage of the 1:4 aspect ratio.