Apple's Security Guide
Apple released a Security Configuration Guide for Mac OS X 10.5, aka Leopard:
The Security Configuration Guides provide an overview of features in
Mac OS X that can be used to enhance security, known as hardening your
The guides are designed to give instructions and recommendations for
securing Mac OS X and for maintaining a secure computer.
NSA Configuration Guide
Another source of advice for securing your Mac is from the NSA. The NSA also provide security configuration guides for hardening your Mac.
The NSA guide for Mac OS X 10.5 is available as a PDF.
Advice beyond these guides depends on how you use your Mac. If it is connected to the Internet and providing services such as file sharing, screen sharing, or remote login then you may wish to investigate updating some open source packages.