Utilize host-based Intrusion Detection Systems (IDS), such as Dragon IDS suite and Comodo

Monitor firewalls for suspicious traffic, particularly outbound traffic to unknown addresses

Implement file integrity monitoring
Secure workstations so packet sniffers, or other malware, cannot be installed

Utilize encrypted protocols or encryption to protect sensitive data

Use packet sniffers legitimately to detect network intrusion attempts or suspicious activity on a network

Ensure that anti-virus and anti-spyware software programs are up-to-date

Routinely examine systems and networks for newly-added hardware devices