try something like
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access plus 1 seconds"
ExpiresByType text/html "access plus 1 seconds"
ExpiresByType image/x-icon "access plus 2592000 seconds"
ExpiresByType image/gif "access plus 2592000 seconds"
ExpiresByType image/jpeg "access plus 2592000 seconds"
ExpiresByType image/png "access plus 2592000 seconds"
ExpiresByType text/css "access plus 604800 seconds"
ExpiresByType text/javascript "access plus 86400 seconds"
ExpiresByType application/x-javascript "access plus 86400 seconds"
</IfModule>
or
<FilesMatch "\.(?i:gif|jpe?g|png|ico|css|js|swf)$">
<IfModule mod_headers.c>
Header set Cache-Control "max-age=172800, public, must-revalidate"
</IfModule>
</FilesMatch>