private function onInit():void{
  var appSO:SharedObject = SharedObject.getLocal("yourappdata");
  if(appSO.size < 0){
    //do your init code
    appSO.data.initialized = true;
    appSO.flush();
  }
}

<mx:Application ... creationComplete="init()"/>
   <mx:Script>
       private function init() : void {
           ...  // put your initialization routine here
       }
   </mx:Script>
</mx:Application>