There is a common technique to instruct the browser to take the new file version: Browsers store different file versions for different GET paramters. So if you include a CSS file like this:
and in the next version with a higher version number (v=22), the browser will use (and cache) the newer version.
In our case we save the current version number (SVN) into the app.yml and we use this number as increasing GET-Parameter to force a cache refresh:
default: stylesheets: [styles.css?v=<?php echo sfConfig::get('app_revision'); ?>]