First you need to enable the mod_expires module:

   1  a2enmod expires

Next add this to your configuration:

   1  ExpiresActive On
   2  # Set Expires header to current time by default
   3  ExpiresDefault A0
   4  
   5  <FilesMatch "\.(flv|ico|pdf|avi|mov|ppt|doc|mp3|wmv|wav)$">
   6    ExpiresDefault "access plus 30 days"
   7  </FilesMatch>
   8  
   9  <FilesMatch "\.(jpg|jpeg|png|gif|swf|bmp|)$">
  10    ExpiresDefault "access plus 7 days"
  11  </FilesMatch>
  12  
  13  <FilesMatch "\.(txt|xml|js|css)$">
  14    ExpiresDefault "access plus 1 day"
  15  </FilesMatch>

Now restart Apache:

   1  $ sudo /etc/init.d/apache2 force-reload

Check that the proper headers are set with Firebug, Yahoo YSlow or Google Page speed.