There are alot of pretty good Linux and Unix based security guides out there like:
http://www.cert.org/tech_tips/usc20_full.html
other things to remember is:
1 fully patch
2 remove any not used services
3 correctly setup all services (like NFS, samba etc)
4 host based firewall and control access with tcp_wrappers and other access controls to the services
5 good practices, like not using root all the time,
6 consider SElinux, for mandotroy access control if you want to really be parinoid and have the time to mess with it / learn it.
I should give you a start.
-Nex6