I tried to keep a journal. However, I get ideas all the time, insights, while driving, sitting at a friends house, wherever. So I'll end up with ideas on 15 different pieces of paper.
I also try to record voice notes into my iphone on Evernote.
When I sit down and start typing the ideas out, they'll be in a folder on my hard drive (dropboxed).
I'm all over the place. Wondering if there is a better system.
These are not always just copywriting ideas, but they will include them. These are marketing ideas, promotion ideas etc.