Take a few minutes and look at the following list to see what’s advisable before you just plunk that box down and hook up the cable to the outside subnet.

  1. Determine It’s Purpose
  2. Do the Installation Right
  3. Setup an Adequate Firewall
  4. Configure TCP Wrappers
  5. Turn Off Non-Essential Services
  6. Secure Your Required Services
  7. Tune Kernel and Networking
  8. Connect to a Router
  9. Update the OS and Apps
  10. Additional Hardening

Additionally, you might want to look at the Tripwire, AppArmor, Bastille and Smoothwall (turns it into a router/firewall) projects for step 10, there are a lot of good ways to increase your security for not much more trouble.  The inspiration for this list came from this article, which goes further in to the explanations for all of these.