To enhance this monochromatic winter vista of Yosemite, I used a split toning effect. I added a deeper sepia tone to the darker tones, and lighter sepia to the brighter areas of the image.
In the analog darkroom, toning was a function of paper, processing and chemistry. Split toning was achieved by altering the process so that highlights were handled differently from the dark areas of the photo; for example, by stopping a chemical bath at an earlier point than normal.
In the Photoshop darkroom toning and split toning are, of course, virtual—like the entire digital monochromatic process. Digital black and white is an aesthetic intention with soul, a willful abnegation of color, and an intentional and anachronistic reference to the historical craft of photography.
You can learn more about toning and split toning techniques in Photoshop in Creative Black & White: Digital Photography Tips & Technques.