These days, I use my iPhone to record most of my writing ideas while on the go, either a brief voice recording or a short note. If I still like the idea by the time I get home, I transfer it to my Scrivener notebook idea page, perhaps expanded a little bit. I try to revisit my idea page regularly and once a week, I take the idea I like the most and expand THAT idea even more.
The goal, of course, is to turn some of these ideas into books.
What about the rest of you? How do YOU record your writing ideas?