Drawing BitmapData to Bitmap

Hey all,
its early and i feel dump right now. I know that i have done this effect with AS2 but can’t seem to get it going in as3.

what i need to do is basically a motion trail on a bitmap.

I want to continuously draw a bitmap onto itself while previous versions are faded into the background, similar to a smoke type effect.

can someone point me in the right direction? probably something silly that i am missing.