The next step is to increase the contrast and define the lines in the image. Keep in mind that this is eventually going to become a mask. The parts of the image we don't want need to be black, the ones we want need to be white (and if we want it semi-transparent we will make it a shade of gray).
There are several ways to make lines more pronounced. First off, I usually try the High Pass filter (Filter>Other>High Pass). This filter makes edges black and the rest of the image gray (try to keep the pixel setting for the filter around 10, but move the slider around to see what works best). Unfortunately for this image, the High Pass filter ended up reducing the contrast too much, so I didn't use it. If I can't get the High Pass filter to work I will usually try to sharpen things up a bit with the Unsharp Mask filter.
After the lines have been intensified, open the Levels dialog box (command-L) and drag the black and white sliders somewhere towards the middle. Pay close attention to the wisps of hair as you adjust the sliders don't move them so far either way that they start to disappear or become ill-defined. You can see the Levels settings I used below.
It's starting to look more masky now.
