1) User accounts should be provided limited priviledges and strengthen ur Passwords.
2) Use security softwares such as AppArmor, SELinux and Iptables (Ubuntu 8.04 default installation already hardened by AppArmor package. You need to sepeartly install and configure SE Linux and IP Tables
# sudo apt-get install SELinux
# sudo apt-get install Iptables
3) Always mount system files and user files on seperate partition. So that, even if anyone partition corrupted, you can retrive the other.
4) Stop Unnecessary services and Ports
5) Provide limitations on using external devices, drives.,
