Make sure that you set up your router to do port forwarding. You will need to assign a static local ip address to your computer and then just tell the router what port to forward and to where. Microsoft Terminal Services communicates over ports 3389, 4194, and 1604. The default is 3389. (Then mirror this configuration on both your network and your friends)
The easy part is now done... Then you need to think about how you will "call" the other person. The easy part is opening up the Remote Desktop Connection... But you will need to find your global ip. If you are lucky, you will have a static ip. If you are not lucky it will be dynamic. To find your ip, go to a site like this:
http://www.whatismyip.com/ Then you and your friend can swap ip's over an IM session or you can go the further step of setting up a domain name.
Also, you both need to run win xp pro to do remote desktop sharing. However as mentioned above you can use VNC the process is similar but the port numbers are different.
Feel free to pm me if you have more questions. I have done set-ups like this for clients and they seem to like it!