How can I keep my Linux secure?
Security is a process, not a permanent state. Once you've taken the initial steps to secure your box, you must engage in regular maintenance to ensure that your box continues to remain secure.
To ensure continued security, regularly do the following:
Keep current with patches - Keep current with your distribution's security updates, and patch on a regular basis.
Monitor Logfiles - Logfiles should be monitored regularly for anomalous events. Monitoring with automated tools is acceptable (Sometimes even necessary!), provided you do a regular manual audit of logfiles as well.
Audit Password Strength - Run a password auditing tool such as John the Ripper every month or so to check for insecure passwords.
Check your binaries - Regularly scan your system for trojaned or otherwise altered binaries using both an integrity checker, and a trojan scanner.
Check for Remote Vulnerabilities- Periodically run a current vulnerability scanner against your machine from another box, preferably one outside of your firewall.