Assuming usage of PHP, you could do this; however, this can be adapted pretty easily to any language.
<?php
$randNum = mt_rand();
?>
<link type="text/css" rel="stylesheet" href="style.css?<?=$randNum?>" />
Here's a demo of it: http://wecodesign.com/demos/stackoverflow-7072702.php
UPDATE You could also set your headers to prevent caching, in the case of a CSS file, you'd need to do something like this at the top of your CSS, which would be renamed with a php extension instead of a css extension:
<?php
header("Content-type: text/css; charset: UTF-8");
header("Cache-Control: no-cache, must-revalidate");
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
?>