Keep your Joomla site updated!
The Joomla Development team are amazing when it comes to responding to security issue regarding Joomla. To fully benefit you need to ensure you are as well. Always keep your Joomla site up to date, and be sure to subscribe to the Joomla Security News Feed: http://feeds.joomla.org/JoomlaSecurityNews You can even subscribe via email.
Next, ensure you patch your installation when security patches are released. The official way to do this is outlined here: http://docs.joomla.org/Upgrading_1.5_from_an_existing_1.5x_version
There are plenty of other Joomla security tips available, but it really comes down to two main things:
- Joomla Hosting
- Joomla and Joomla Extensions Security
So, you've read this far already.. have you subscribed here yet: http://feeds.joomla.org/JoomlaSecurityNews ?