I have a high tech system for recording and cataloging the paint schemes I use on my miniatures. I scrawl them out onto a piece of paper and then stack it on my desk! It actually is quite useful, although I forget to do it sometimes. I record mixtures, or any unusual techniques I did, as well. For colors not available anymore, I'll write down the mixture for something similar when I need it again. It comes in handy when you return to a project after a couple of years away from it.
Not that it's really that interesting, but I wanted to take some pictures as a backup in case these ever got lost. I'm not sure how far back this goes, but I'd say at least five years, probably more on the order of seven.