W .htaccess dodaj to (dla różnych formatów plików możesz ustawić różne wartości).
Kod:
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault A86400
ExpiresByType image/x-icon A2592000
ExpiresByType application/x-javascript A1209600
ExpiresByType text/css A1209600
ExpiresByType image/gif A1209600
ExpiresByType image/png A1209600
ExpiresByType image/jpeg A1209600
ExpiresByType text/plain A86400
ExpiresByType application/x-shockwave-flash A2592000
ExpiresByType video/x-flv A2592000
ExpiresByType application/pdf A2592000
ExpiresByType text/html A86400
</IfModule>