With many of my clients I use PCHelpware.
http://www.uvnc.com/pchelpware/
It's possible with this to create an exe file for the client that has your IP hard-coded. When they run the file it attempts to connect to the viewer on your machine. This one-to-one relationship takes away some of the risk of having a service on the client machine waiting for a connection. Also, because it's active from both ends it will work through a firewall without having to forward any ports.