The 960.gs helps me keep my css clean because I'm able to establish the bulk of the site design using predefined grid classes as opposed to custom defined classes which are tightly coupled to a particular layout.
This may not be as popular but I also started writing my styles on a single line (eg #header {height: 100px; background: silver; padding: 0 0 20px 0;}). I've found it much easier to scroll through my stylesheets and find the classes I'm looking for.
I would recommend using code folding over single line CSS which can easily get cluttered. By using cold folding you get the benefit of having one line per style, but you can jump down into a particular declaration and still have the readability of multiline styles.