I think that this is the one feature that I've been waiting for more than anything on Google's mapping software. If you go to http://maps.google.com/ and enter a start and end point it will map your route for you. But say you wanted to go via Las Vegas to get there. (Hey, who doesn't want to go via Vegas?) So you enter Los Angeles as your start point and then enter Las Vegas as your destination. Google maps it for you.
Then you might, if you look very carefully, see an "Add Destination (new)" link in the bottom left corner of the screen. Click this and add San Diego and hey presto, it will show you how to get from Los Angeles to San Diego using a short cut via Vegas. (If Vegas isn't a short cut then I don't know what is.)