An interesting technique that makes creative use of Photoshop's Layer Style function to give the illusion of depth and texture to brush strokes.

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Painting with Depth in Adobe Photoshop by Dave Nagel

Part 1: Basic use of contours in painting
http://www.macdesignpro.com/articles/viewarticle.jsp?id=34093

Part 2: Using lighting, range and contour options to affect the appearance of depth
http://www.macdesignpro.com/articles/viewarticle.jsp?id=34199

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Links courtesy of Phosphor at Adobe.com.