Apple Remote Desktop
Here's one I didn't know.
Make sure that on BOTH the system that you are trying to control, as well as the system that you are controlling from - that the firewall on your router is set to port forward UDF port 3283 to the appropriate computer!
What does that mean?
Go into your router and look for the 'forwarding' or 'ports' option on it (my netgear calls it port, and the lynksys at work calls it forwarding but both put them under 'advanced') and add the IP address of the machine that you want to control, or are controlling from and make sure that you point the ports to them.
As you can see from my setup below:
I've got two different machines that I'm serving different services off of. One of them is at .2 and the other is at .6
So, depending on what it is you are requesting (jabber, or web browsing, or Apple Remote Desktop) you're going to end up on a different machine.
Make sense?