private static const SCALE_DROP:Number = .5;

public function resample(bitmapData:BitmapData, newWidth:uint, newHeight:uint):BitmapData
{
    var finalScale:Number = Math.min(newWidth/bitmapData.width, newHeight/bitmapData.height);
    var finalData:BitmapData = bitmapData;
   
    if (finalScale> 1) {
        finalData = new BitmapData(bitmapData.width*finalScale, bitmapData.height*finalScale, true, 0);
        finalData.draw(bitmapData, new Matrix(finalScale, 0, 0, finalScale), null, null, null, true);
        return finalData;
    }
   
    var drop:Number = SCALE_DROP;
    var initialScale:Number = finalScale;
    while (initialScale/drop <1) initialScale /= drop;
   
    var bd:BitmapData = new BitmapData(Math.ceil(bitmapData.width*initialScale), Math.ceil(bitmapData.height*initialScale));
    bd.draw(finalData, new Matrix(initialScale, 0, 0, initialScale), null, null, null, true);
    finalData = bd;
   
    for (var scale:Number = initialScale*drop; Math.round(scale*1000)>= Math.round(finalScale*1000); scale *= drop) {
        bd = new BitmapData(Math.ceil(bitmapData.width*scale), Math.ceil(bitmapData.height*scale));
        bd.draw(finalData, new Matrix(drop, 0, 0, drop), null, null, null, true);
        finalData = bd;
    }
   
    return finalData;
}

http://jacwright.com/blog/221/high-quality-high-performance-thumbnails-in-flash/

High quality, high performance thumbnails

ActionScript 3