Here is my time-lapsed video of the process I go through when painting a portrait in Photoshop CS3 with my WACOM tablet, using mainly the airbrush and some customized texture brushes.
Also a nice demonstration of David Nagel's Photoshop Skin texture brushes
which I cherish so dearly! They're fantastic for blending tones together while keeping the texture of the skin intact and looking real.
Real time- around 4-5 hours.