Steps to harden your Debian Linux installation:

http://www.debian.org/doc/manuals/securing-debian-howto/

The following site has some scripts for securing Linux to the DISA STIG standard. These were written for RHEL 5, but many of them should would for Debian as well.

http://oss.tresys.com/projects/clip/browser/trunk/RHEL5/scripts/stig-fix?rev=110