reset.css - I use eric meyer's
elements.css - Global defaults for things like body,
h1, h2, h3, p, a, input, strong. All
or almost all selectors in here are
just tag names.
layout.css - Just sets up the global layout with
containers: e.g. header, footer, left
column, right column.
blocks.css - Reusable chunks.
In addition to that, I use a separate file for each "page type". For example, 'article.css', 'index.css', stuff like that. Some larger sites merit additional files like splitting off 'forms.css', 'tabular.css', etc.All files get concatenated and minified before serving, obviously.