hardening the PHP-CMS Joomla!

I’m not a big fan of Joomla! but a client wanted to use it and so I had a closer look on it, to make it a bit securer.
For the moment I work with the 1.0.14 version and read that the Joomla1.5 work with safe mode on and has some nice security features.
Here are some tips which you can also use if you aren’t a Joomla head.