Port forwarding

This should be so easy, but I've made several attempts and each have failed, so I need to start keeping track of what I have tried so I don't repeat myself.

The goal is to configure my iptables firewall to pass traffic from the internet through my server to a NATted box on my internal network where I'm running a service of some kind.

References:

• http://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#IPHEADERS
• http://www.ridinglinux.org/2008/05/21/simple-port-forwarding-with-iptables-in-linux/