Eitehr dupicate a hicontrast channel or use caluclations to create a new one
Then adjust levels on the channel to create a good matte
Then use a command click ot load channel to make a slection
Add a layer mask etc
THROUGOUGHLY covered with hands on exercises in both my Photoshop Books
Understanding Adobe Photoshop (for general audience)
Photoshop CS for Nonlinear Editors (for video audience)