I actually write ALL my ideas down! I probably have a hundred by now. But you are right, there's always one or two ideas that keep nagging at me, and right now I'm pursuing an idea that although it wasn't mine, right now I simply cannot not put it aside.

My filter for writing it down is simply if an idea represents an actual problem that a real person is facing. In other words, I don't write down an idea that I simply think it's cool, but when I hear about someone facing a real problem.