Garrett Dimon’s article Architecting CSS contains some useful guidelines for organising CSS. It can be really tricky to keep track of what every CSS rule does when a site grows large or a design becomes complex, so tips like these are good.
After spending too much time trying to figure out what does what when going back to make a few simple changes to an old site, I’ve become motivated to improve in this area. Tidy my CSS files are not. At least not as tidy and structured as they could (and should) be.