I was exposed to the XP idea of using index cards as a preferred tool back in 1999. For a while, I still preferred whiteboard work to cards, just for the large visibility. But I quickly came to recognize the simple power of things like deletion-by-ripping, rapid ordering, arbitrary grouping, simplicity of shared communication and agreement, and also the power of mobility--they go everywhere, even through an airport metal detector. (Water, often found in washing machines, is a bit of a problem.)
Once a week or so, I run through the dozen or so cards tucked in my jeans or shirt pocket. I scratch out useless or already ingrained information, consolidate the really useful information, and delete the completely used or otherwise useless cards. So much quicker than a BlackBerry!
I keep a small number of reference cards handy; one is a vi card. Despite what you might think, the vi card changes frequently. I'm often seeking better ways to do things, and of course note those on my vi card. But once I do something often enough and thus ingrain it (I now know to rotate my split screens with ctrl-w-r), it doesn't really need to appear on the card. I scratch it off, and after enough scratchings or when there's no room left, I rebuild the card.
Apropos of little: Despite our best attempts otherwise, many of us end up being just like our parents. A few years ago, and by then well addicted to the cards, I caught myself in the mirror. I realized that I was like my father in many ways: political philosophy, appearance, and demeanor. I had to laugh when I noted what made the transformation seem complete: scribbled notes on a stack of index cards stuffed into my shirt pocket, sitting alongside a couple pens. I am my father.