Force CSS Recache
css, stylesheet
Solution
The single surest way to ensure that everyone reloads your new file is to change its URL. A reasonable way to do this without actually renaming it is to tack on a random string:
<link rel='stylesheet' href='/mystyles.css?random=87365835'>
That's using a sledgehammer to crack a nut, and bypassing all the HTTP mechanisms that exist to do this, but it will absolutely work everywhere, whereas the other techniques all have caveats.
Edit: @jitter: You don't have to change the URL automatically for every request - you could base it on the timestamp of the CSS file, or even just change it by hand when you want to ensure that the CSS gets reloaded.
Problem
Is there any way to determine or force a CSS file to be re-cached if the user is operating off an old file?