Setting the cache expiry period of your visitors' browsers allows storing files in their browsers, so future downloads may be avoided and the download speed becomes increased, therefore gaining a better valuation from Google. Use the following rule in your .htaccess file to set the cache expiry period of your browser:
<FilesMatch "\.(php|html|htm|txt|html5|jpg|png|js|css)$">
Header set Cache-Control "max-age=604800"
Header set last-modified "Tus May 15 13:00:00 EDT 2012"
</FilesMatch>
<FilesMatch "\.(pl|cgi|spl|html)$">
Header unset Cache-Control
Header unset Expires
Header unset Last-Modified
FileETag None
Header unset ETag
Header unset Pragma
</FilesMatch>