The solution is to intelligently cache stylesheets when they haven’t changed, but to reload them when they’ve been updated utilizing the PHP file modification time function filemtime(). Some CMSes do this natively (like Expression Engine), but WordPress doesn’t— yet.
You can add this functionality to your theme by modifying the stylesheet link in the header.php theme file as follows:
<link rel="stylesheet" type="text/css" href="<?php bloginfo('stylesheet_url'); echo '?' . filemtime( get_stylesheet_directory() . '/style.css'); ?>" media="screen">
<?php $cachebustFile = '/your-subdirectory/your-file.ext'; ?> <script src="<?php echo get_template_directory_uri(); echo $cachebustFile . '?' . filemtime(get_template_directory() . $cachebustFile); ?>"></script>
Note: If you have downloaded my KM-H5 template theme after Dec. 31, 2011, this feature is already included in the core theme files.