General Steps for Hardening Security on a System
Keep track with the latest vulnerabilities and patch your system
Be aware about the services running and disable unneded services and ports
Protect running services with tcp_wrappers if possible
On the Internet, use only secure services
Protect running network services with netfilter firewall
Verify open ports and firewall rules with port scans
Protect sensitieve data exchange over the internet with encryption
On multi-user systems, protect access to user accounts
(password hashes), set correct permissions on home directories, set disk quotas, establish user policies regarding passwords, applications, data, etc.