Basically the gist of http://docs.joomla.org/Security_Checkli … omla_Setup is as follows: 

Security Checklist 4 - Joomla Setup 
1. Install official versions of Joomla 
2. Change the default administrator username 
3. Protect directories and files 
4. Adjust file and directory permissions 
5. Remove unneeded files 
6. Turn Register Globals Emulation OFF 
7. Check for extension vulnerabilities 
8. Remove junk files 

Source: http://docs.joomla.org/Security_Checkli … omla_Setup