The users browser don't cache static content, if your web-server don't sets the right expire or max-age headers. Drupal sets these headers for static content in the .htaccess file that comes with Drupal, if you have the expire Apache extension enabled. You can use http://www.webpagetest.org or a Firefox plug-in such as YSlow or Page speed to test if your server sets the headers.
If your server do not set the headers, try enabling mod_expire by executing the commands below as root on the server.
~$ a2enmod expires
~$ /etc/init.d/apache2 restart