To enable caching in the browser, add the following code in .htaccess:
<FilesMatch ".(flv|gif|jpg|jpeg|png|ico|swf|pdf|woff|css|js)$">
Header set Cache-Control "max-age=604800"
</FilesMatch>
In which you must list all the file extensions (gif|jpg|jpeg|png|...), to be cached.
max-age=604800 - time that you want to cache files. Is given in seconds (in this case 7 days).
Possible mistakes
If you add this block in the .htaccess file and you got an error 500 Internal server error:
Most likely you do not have a module headers.load, for that would enable it you need to enter in sictemu as an administrator and run the following commands:
cd /etc/apache2/mods-enabled/ ln -s ../mods-available/headers.load headers.load sh /etc/init.d/apache2 reload
Create a symbolic link to the mods-available/headers.load in mods-enabled and restart Apache.