Turns out people use all sorts of different systems, usually including some sort of date reference, or abbreviations for clients and projects.
I like many of the points made by nex, who remarks that good, modern desktop search has made filenaming less important:
Use a system that will work with your laziness, in spite of your laziness, something that doesn’t rely on you forcing yourself to play by tedious rules
I couldn’t agree with that more. Personally, I don’t bother with dates in filenames, since the Created and Modified dates created by OS X are enough. I make use of a handful of prefixes (“orb” for O’Reilly blog, “mu” for MacUser, and so on) but otherwise leave things as simple and as easy-to-remember as possible.