If you are like me and you find yourself constantly scribbling on napkins trying to think through ideas, then you can do no better than to keep them in a notebook. If you don't already have some notebook brand your tied to, then try these Scratch notebooks by Draft Supply Co..
That's what I just did. (In dot-grid if you must know.)
Updated (2016-12-28): The link is dead, so I've reverted to a version stored on the Internet Archive.