Yep I think the XML file is the good approach.
I personnaly use a command included in the StartupCommand MacroCommand as the last entry to start the loading of the config. When the loading ends I create a ConfigProxy which is later responsible to give access to configuration. It defines methods as ConfigProxy.getString(propertyName), ConfigProxy.getInt(propertyName), etc. when I need my service URL I simply call something like : facade.retrieveProxy("CONFIG_PROXY").getString("URL").
If you have more than one files to load, you can use the PureMVC loadup utility to help in loading XML and other dependencies : http://trac.puremvc.org/Utility_AS3_Loadup