I generally need something external input. Could be simply random words that I can use, sometimes I go browsing art work that inspires me to write a story around it. Sometimes I want to fill in what happened in a Fade to Black moment, or rewrite a story that in my mind simply makes no sense the guy did not abuse the girl. Sometimes it could be just an innocent conversation, where just a word, or a thought, strike me and stays with me longer, or just seeing a random person in my commute to and from work.
Sometimes its an already written story, that I simply want to add Dragons to, as their is never enough of those. Or a story that I can drag into and fit into my massively expanding alternate universe.
Ideas come and go faster than I think sometimes, and it seems most are gone before they get written or even remembered.