I have a personal wiki. I jot down ideas on a page or two, use tags to get some idea of what a given page is about.
Sometimes an idea is more about how to approach something like player involvement or a generic way to handle resources. Those things go on their own page, but I make sure to keep a link up.
I also keep a small number of blue stickie note with me, and write down anything I think of, even if I've written it down before.
Its those stand out things that shine or draw you or excite you that you want to capture and try to retain.
