Inspiration
I took a trip to the NY Society of Illustrators to visit the 2009 Illustration Annual Exhibition. While I was there one piece in particular caught my attention. It was called "Gibson Girl" by Sterling Hundley. Much like Charles Dana Gibson's illustrations, the woman in Hundley's illustration had the famous hairstyle most commonly seen in Gibson's work. So this inspired me to follow in those footsteps and create a portrait of a woman with this hairstyle.
Process
Every one of my illustrations starts out with a very tight drawing on paper, usually from some sort of photo reference or previous sketch. It is at this stage where I plan out the composition, lighting and values of the piece, as seen in Fig.01. This stage usually takes a couple of hours. After this the drawing is scanned at 300DPI and taken into Photoshop for the digital paint process.

Fig. 01
In Photoshop, I usually start out by blocking in solid colors in the shapes of the main forms (Fig.02). After this I set the drawing layer to Multiply, at an opacity of anywhere from 50%-75%. This all depends on the overall value range of the piece: if the piece is very dark, like this one, I have to set it at a higher opacity; if the piece has a very light value range then I usually set the drawing layer at anywhere from 25%- 40% (Fig.03).
After the initial colors are blocked in, I begin by painting in the darkest shadows to kind of set the tone; now I know not to go any darker than this anywhere else on the face (Fig.04).

Fig. 04