Giving 'weight' to a movie clip

A tricky one to explain, this, as I can’t post example AS or SWFs, but here’s what I’m trying to do…

I’m creating a game which features a top-down view of a character carrying a sack over their shoulder. The character moves by following the cursor and is always rotated to face it. I want to give the sack (which is a movie clip nested in the character) some ‘weight’ by having it rotate slightly more slowly then the character - i.e. when the character turns, the sack takes a moment to catch up. At the moment its orientation relative to the character doesn’t change and the effect is a bit stiff.

The code I’m using to rotate the character is along the lines of

function pointAtMouse(target) {
var dx = _xmouse - target._x;
var dy = _ymouse - target._y;
return Math.atan2(dy,dx) * 180/Math.PI;
}

Can anyone suggest how I can make the my sack ‘swing’, if you’ll pardon the expression?