Add Alternate Access Mapping for a WSS web application

You would think that the Alternate access mappings link under Global Configuration on the Operations page in Central Admin is the place to start with if you need to add an Alternate Access Mapping (AAM) for a web application. Nope. You start adding an AAM by extending the existing web application (Create or extend Web application link on the Application Management page). On the "Extend Web Application to Another IIS Web Site" page, make sure the existing web application is selected, and make sure select a value in the Zone dropdown box at bottom of the page that is different from the existing web application. Once you are done extending the web application. Go back to the Alternate Access Mappings page again and you will see the newly extended web application is there.

This TechNet article does an excellent job of walking through the step by step process of setting up an AAM (http://technet2.microsoft.com/windowsserver/WSS/en/library/c8ccffce-5162-46af-a3ef-1d7914e8efee1033.mspx?mfr=true).