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If you are having problems, all you have to do is hit ctrl + F5 to do a full refresh, plus make sure your caching is set correctly, but a ctrl+F5 does a full reload of all elements, including those pesky css files.
I second Matthew's advice here, I just had a major issue with a new site going up, but still using the old css file, a simple renaming resolved the problem instantly of course. I use a similar method to Matthew, a sitewide template that allows easy changes to the <head> area.
I'd forgotten that while my browsers are all set to refresh correctly, the average browser is set to only refresh when it thinks that file is new, and CSS files seem to pose special issues, even for IE. This seems to be an especial problem when you use the @import method to bring in the stylesheets, one tester could not get the new style sheet up at all, even with a full refresh, ctrl+F5, on Firefox 1.0.2, which is a mystery to me since my browsers all refreshed it fine.