I would say paper - all kinds of paper. I have a small cupboard dedicated just to fancy papers! Whenever I feel to 'do' something, I just remove those papers and start working. I'll either make a card splashed in a lot of colors or a pocket diary. Oh, it's so amazing!
I was just thinking about this tonight, that is, the creative spark versus following through on a project. What is it that will help more ideas come into realization? I think that your method of using lists and writing out the steps is a wonderful key in this process.