Quick example of how to set default expires headers.
Put this in your .htaccess file:
<IfModule mod_expires.c>
ExpiresActive on
ExpiresByType image/gif "access plus 10 years"
ExpiresByType image/jpeg "access plus 10 years"
ExpiresByType image/png "access plus 10 years"
ExpiresByType text/css "access plus 10 years"
ExpiresByType text/js "access plus 10 years"
ExpiresByType text/javascript "access plus 10 years"
ExpiresByType application/javascript "access plus 10 years"
ExpiresByType application/x-javascript "access plus 10 years"
#ExpiresDefault "access plus 1 days"
</IfModule>