For maintainability I break them up in the same way for all the websites.

• Base is for the background colors and images used on all pages and page structure.
• Menu is for the site navigation menu and right side of the page.
• Tutorials for page specific styling. I always define one for each page (or in the case of the tutorials, one for the group).
• Base_IE has everything from all pages that need to be different for IE.
• Base_IE7 is anything else needed for IE7 that can't be included in Base_IE.

In addition, within each stylesheet, I always define the styling in the same order as the HTML appears on the page. I never have to hunt around for the element I need to modify. It is a BIG time saver.