Photoshop Tip: Channels for selections
Tips & Tutorials, by Mark White
November 16, 2012
In the Channels palette in Photoshop look for the channel (from R,G and B) that gives the most contrast between the subject and background
Drag this channel on to the Create New Channel button to duplicate, and use Levels (Cmd/Ctrl+L) to increase contrast.
Paint with the Brush tool using black and white to add or remove parts of this duplicated channel. Cmd/Ctrl-click on its thumbnail to generate a selection.