Here is a nice article in Digital Web Magazine on Architecting CSS.

With nearly ubiquitous standards support among modern browsers, we’re turning to CSS to handle presentational heavy lifting more than ever. The more we rely upon CSS, the larger and more complex CSS files become. These files bring with them a few maintenance and organizational challenges.

Gone are the days of creating a single CSS file and dropping in rules as needed. As we build new sites, it is necessary to spend time planning how to organize and structure CSS.

Following the tips in this article should save you quite a bit of time during initial design and when going back to revise later.