private var container:MovieClip = new Container(); 
private var content:MovieCLip = new Content(); 
// where container is your stage, and content is the material you want to center

function init() {
 
stage.align=StageAlign.TOP_LEFT;
stage.scaleMode=StageScaleMode.NO_SCALE;
 
stage.addEventListener(Event.RESIZE, updateSize);
stage.dispatchEvent(new Event(Event.RESIZE));
 
container.x = 0;
container.y = 0;
 
}

function updateSize(e:Event) {
 
// make container width/height equal to stage.width/height
container.width = stage.stageWidth;
container.height = stage.stageHeight;
 
//center content
content.x = stage.stageWidth/2 - content.width/2;
content.y = stage.stageHeight/2 - content.height/2;
 
}