Arno Nel links to a pretty good CSS article, Architecting CSS, which is about how to organize and maintain your CSS code.
I think for the most part the article has some great advice. I don’t necessarily agree with the first part about file organization. The author suggests splitting up style rules by area of the page or of the site into multiple CSS files. I personally don’t like splitting up style rules across multiple files. I never practice this myself (unless it is to provide multiple skins for one site). But I have worked with the styles for both SharePoint and ProjectServer, and I find it quite aggravating because you have to search and look in multiple files to find a style rule. If you follow the rest of the advice in the article in regards to organizing your CSS code within the file itself, you shouldn’t need multiple files for organizational purposes (for example he cites keeping footer styles in its own footer style sheet file). If you group, alphabetize, label and clearly define what style rules affect what section of the site, it is easy to maintain in one file.
Another tidbit, to create comments in a style sheet, comment the code much like how you would do it in HTML, but use the characters /* to start and */ to end. For example:
/* Here is my comment */