I think every writer around keeps a journal of some sort filled with ideas. I keep more than one, if truth be told, and a couple electronic ones, too.
Picture this: an idea you once had a year or three ago reasserts itself in your mind. You turn it over a few times, checking to see why, perhaps, it returned to the forefront of your mind. Satisfied that you might want to work on it again, you face a dilemma: do you re-read your old notes from way back when or do you charge forward, assured that either you will remember all the ideas you had about this topic the more you write OR do you let new ideas filter in?