Hello, Please can someone help me...?

I have a Flash Movie which has an image that zooms in/out with buttons and can be reset using the following code.

Code:
zoomPlus.onRelease = function(zoomIn) {
    if (test_shirt.shirt._xscale == 400 && test_shirt.shirt._yscale == 400) {        zoomIn.enabled = false;
    } else {
        test_shirt.shirt._xscale += 20;        test_shirt.shirt._yscale += 20;
    }
};
zoomMinus.onRelease = function(zoomOut) {
    if (test_shirt.shirt._xscale == 100 && test_shirt.shirt._yscale == 100) {
        zoomOut.enabled = false;
    } else {
        test_shirt.shirt._xscale -= 20;
        test_shirt.shirt._yscale -= 20;
    }
};
reSize.onRelease = function(reSize) {
    test_shirt.shirt._xscale = 100
    test_shirt.shirt._yscale = 100
    test_shirt.shirt._x = 0
    test_shirt.shirt._y = 0  
};
The effect is not very smooth so my question is how do i do this with easing.

Is there anyway someone could show me how to adapt my script so the movieclip would ease as i zoom in and out?

Thank you in advance!!!