I upped the contrast in Photoshop and posterized it to define value changes.

I transferred the photo to Illustrator and used the pen tool to trace the definitive value groups, and filled them with one of five greyscale values. Once the basic five were on there, I began to play with opacity to give the illusion of more values and three-dimensional form.

Once I had the monochromatic vectors in the program, I developed this color scheme on Kuler, which was convenient because I was limited to five values anyway. After I changed all of the greys to these five colors, I adjusted some of the less opaque vectors and called it complete.