I discovered notepad really helps me when new ideas sweep me off my feet. I write down my idea and a few key nots about it. That way I don't lose the idea and spend a lot of time thinking about it. If the idea still sounds good after a month or so, then I might spend a few more hours clarifying the idea into a more concrete concept.
I've developed this habit, too. One problem I've run across (which I'm thinking you also might have) is I end the day with a whole bunch of notepads open and it is a pain to save/close them.
If you have that problem, too, you should check out a program I wrote called Notepad Consolidator (https://github.com/jaredsohn/NotepadConsolidator) that writes the contents of all open untitled notepads to an autonamed file.
I converted the lean startup worksheet into a Freemind mindmap. For new ideas, I simply open this mindmap template, fill it out, and store it in my main SVN repository under ideas. It makes documenting a new idea and running a quick feasibility analysis a matter of a few minutes.
