Often, the greatest impediment to sensitive and elegant retouching is my lack of good taste. To help find what styles people like, I use Facebook to get opinions from friends. Recently, I posted a series of three images of Andrelica (semi black and white, 100% black and white, and 100% color), and asked which version people preferred. The result was that the semi black and white was preferred by far over the other two versions.
I find that the "best" (if such a thing exists) percentage of black and white to color varies depending on the color intensity of the original image. Here are three images with varying percentages of black and white:
The above image of Andrelica is dark to begin with, so a higher percentage (30%) of color needs to be shown.
The image of Laurel has a somewhat medium color brightness, so 80% black and white and 20% color is a good choice.
The original image of Charissa was very bright and vibrant, so 90% black and white and 10% color works best here. This simple technique is a pleasing way to tone down those over-saturated high dynamic range (HDR) backgrounds in glamour composites, such as the above image of Charissa.