What I did was duplicate the layer, used a combination of hand painting with the brush and using curves to adjust it until I had the girl all white and the background black using the same methods as you would with alpha channel selections, then did a select all, copy, created a layer mask for the original picture, went to the channels palette and pasted it on the layer mask channel. You can then delete the working duplicate if you want.
Hopefully that will get you going since it sounds like you've tried working with alpha channel extractions before, it's basically the same method. Places like her left arm (viewer's right) you'll need to hand paint the edges because there is another person's arm that interferes... and near her head you might want to knock a couple places out by hand... but overall, just curves was all I needed. I did it on a few passes just to make it easier for me... ie... adjusted with curves, hit ok, curves again. The only reason was that because each time the curves is okayed and restarted, the curve line returns to normal and you have more fine control of where to move it, in effect giving you more control of how you fine tune the curve.
