I keep a notebook(s) in which I write down anything I find interesting at that time, or even new Japanese words/characters(単語). I enjoy thinking about the structure of the book and how the narrative moves the reader along. As a kid I was a voracious reader because at boarding school (should that be boring school) there was not much to do, there was no TV, no computers so we made our own entertainment, such as playing Mahjong.
These two traits, note-taking and reading, have been invaluable in writing. Also, so far, I have tended to write about unrelated matters. Agatha Christie just writes detective stories, Martha Stewart cookbooks; me, I write about whatever I find interesting.