Photoshop

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Anatomy of a picture by Will Kramer

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Ok.. another shadow layer, this one for the shirt. This was done after most of the shirt was completed (next layer below). Same as the jean shadows, but for one big difference. The shadows for the strings across her chest. That was freehand airbrush. You can't just drop-shadow the shirt... shadows don't move in straight lines when you're talking cleavage. It took a couple tries (ctl-z!!!... eraser!), but I got to the point where I thought it looked realistic.
Now the main shirt layer. This is some work. A couple layers, much experimentation until it looks right. I try to pay attention to how clothes form around a person. Look around, look at yourself.. see how the fabric stretches.

I never worry too much about getting color just right. I can always colorize, adjust, and tweak a few things later to get exactly what I'm looking for.
Another shirt layer. I know you don't see much difference, but this is a duplicate set to 'multiply' at 16% to darken it a bit. I know I could have just adjusted the brightness and contrast of the original layer, but here's the thing: with this layer, I can go back and adjust the opacity of this layer at any time, changing the darkness of the shirt. Set to 'hard light' or 'overlay', I could have adjusted the contrast.
Here's the layer of 'dirt' for the shirt. Same as the jeans, but more brown. Again, the layer was set to 'multiply', this time at 63%.
Moving up the layers, we've come to the hair shadow. I've erased the part I don't need to see. Again, I have to be careful, because it can't be just a 'drop-shadow'. The way the hair throws shadows across the curves of her face can be much different. I added a bit more with the airbrush on her neck, for example.