JOT IT DOWN. I love sketch books. My grandmother use to always carry notepads, sketch books, pens, and pencils with her everywhere she went. You never know when inspiration will hit you.
My friend and fellow designer, JANE OH not only jots down an idea or thought, but she also writes the time it happend. I thought that was a brilliant and interesting thing to do.
I am still sketching and reworking my collection. Just when you think you have it all figured out, inspiration STRIKES AGAIN :)