I learned early on not to trust my memory--especially if it was something that came to me late at night as I was trying to fall asleep.
I've jotted ideas down on receipts, napkins and kids spelling lists. When I am in the zone and spending a lot of time thinking of ideas they often come to me at inconvenient moments. If I don't write them down, I don't always remember them. (Even when it is a book title that I will never use just because of a ficticious storyline I made up to tease a friend).
Another handy tool for me is a notebook. I don't have a laptop, so a notebook is useful when I'm sitting in the school parking lot or waiting for kids at the orthodontist. Then I just have to fight with them to get on the computer to transfer my work.
Even if it's an idea I might not get around to finishing anytime soon, or not at all, it's nice to have reference other than my memory which ain't what it used to be. ;-)