The whole point of port-forwarding is to tell your router that when a request comes from the outside world to your external ip address, looking to access a certain port, it points that request to a certain machine on your network (ie 192.168.x.x)

Visit http://whatismyipaddress.com from your browser. That is your external ip address.

Then, you can goto http://DynDNS.com and set up a url that forwards to your ip address. There is software that you can download from them, something like "dyndns updater" and it will change where your new url forwards to every time your ip address changes.

SO,

if your connecting to, say your.ip.add.ress:3389 right now, you can assign yourdomain.whatever.com (which is free to set up) to point to your.ip.add.ress.

The result is that you can connect to yourdomain.whatever.com:3389 and it will always point to your port on your machine, even if your ip address changes.