Remote control of a Mac that's behind a NAT

I'll soon be traveling to set up a Mac for my elderly mother. I'll need to later be able to control it from outside the network. I already use Apple Remote Desktop to control other Macs on my network, but i don't know how to penetrate the remote network to get to her computer. What do I have to do to get her router to let me connect to it? I don't know what type of router she has, but if need be I will be installing an Airport Express or Extreme instead.