Whether you are using Debian, Windows, Snow Leopard or any other operating system, there are standard practices for securing your environment to limit exposure to your critical systems. They are not hard to follow. Especially when someone provides step by step documentation on how to secure every operating system on the planet. That is exactly what the folks over at the Center for Internet Security have done.
The Center for Internet Security have provided a variety of resources free to the public detailing best practices for securing a variety of systems. These best practices detail warning banners, logging, file/directory permissions and access, kernel tuning, port blocking and a variety of other security measures that any system administrator can easily follow. Provided in the documentation are scripts which walk you through the process of securing your environment.
For network administrators, many of these best practices can be applied through policies throughout your network to ensure that end users have the level of access they need while providing protection for your valuable infrastructure resources.
Check out the benchmarks over at http://www.cisecurity.org to find out how to best secure your environment.