I have an index card stuck in my copy of Anne Lamott's Bird By Bird. In that essay, she exhorts the reader to write down the ideas and thoughts and clever lines of dialog and whatever else they want to remember on a handy index card (carry one, folded, in your back pocket so you don't look bulky). I just reread it out loud to myself, and now I kick myself for not having recorded myself to listen to it again (not for you all mind you, just for me).
Bird By Bird is one of my favorite books about writing, and it turns out that it reads really well too – marvelous.
I don't carry index cards with me all the time – for a while I did, and now I've stopped, more or less. That should be fixable, and the completely inexcusable case where I don't even have a pen with me is fixable too.
The digital equivalents I have for them are either pages in my personal wiki (again, not for you, just for me) or posts like this one in whichever blog seems the most relevant. In both cases the Socialtext and Typepad support for Blackberry devices are key to having some reasonable web-based system that's always available especially when I'm on the bus.
The most fun I've seen with index cards as a system has to be hawkexpress's PoIC ("Pile of Index Cards") system, which combines ideas and methods and rigor in a way that to me is distinctly Japanese. Aki's PoIC system is new to me, and you can read more about it at next-action.net. jazzmasterson's "notes to self" capture some of the same ideas.
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Edward Vielmetti's Wordpress weblog 