When you choosing the channel for creating the mask, what you looking for is highest contrast. In this particular case it was red channel, very offten its blue channel.

By duplicating the channel (drag & drop to new channel icon), you simply preserving original channel and using this copy for its purpose. You can have more than one copy of course for complex masks.

With levels you just pushing that contrast little bit more to get your self job easier.

Curves are far more advanced than levels, because they offer perfect control where you want to increase contrast.

Overlay mode for brush (not in layers palette) allows you to paint more freely. Its important which color you pick at start. If you pick black, press mouse hold it and painting the mask it will completely ignore the white area. Thats why its perfect for hair.

Areas that simply cant be masked with all the above (inside face) are fixed with any of selection tools and by filling the pure black.

With mask you didnt delete anything, just hide it. So there is no need to erase anything, just click on the mask in layers palette and paint with black to hide what you want.

Tihomir

Open your mind and share the knowledge!