If you don't want to muddle around with router settings, you have some other options. One of those is Edovia's $30 Screens. Screens is a screen sharing client, like OS X's built-in version, but it has additional advanced features, such as increased security and more configuration options.
However, one of the most useful features for troubleshooting remote computers is Screens Connect (see Figure 4). Install this free app on any machineMac or PCand it'll cut through all the shenanigans otherwise involved in network configuration. Just make sure that both the remote computer and the client computer are logged in to the same Screens Connect account, and when you launch Screens on the client computer, the remote computer should show up automatically under Screenss Discovered Computers section. Connect to the remote computer, enter a username and password, and you're all ready to go.