Story ideas come and go so quickly. Some get written down, but most pass through my mind and vanish, unwritten and forgotten. Some even occur in a dream I had. So I've begun writing down every new story idea that comes to me. Even if I don't think an idea is any good, I write it down and brainstorm it anyway.
When I go back and read through them, many ideas get crumpled up and thrown away, leaving me wondering, What was I thinking?! Very few actually make the cut for a good storyline. So I try to keep my ideas - both good and bad - flowing no matter what. You never know what they can lead to (hopefully a publishable story!).