The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 83

Figure 2.8 Add a layer mask and paint with a black brush to hide areas where the
background shows through.
Tip
Don't use a brush that is totally soft (0% hardness), because when you get near
the edges your brush strokes will spill over and remove more than you want to.
The fantastic thing about creating composites with this technique is how every
single fine hair is included simply by changing the blend mode. You can clearly
see this in Figure 2.9 .