From Photoshop Finishing Touches
Katrin Eismann showed a technique at Photoshop World in her awesome class "From Color to Black and White and Back Again" that used a Hue/Saturation and Selective Color adjustment layer to convert a color image to black and white. I thought it was a great concept and adapted it slightly here, using a Hue/Saturation and Color Balance adjustment layer.
From the adjustment layer popup menu, choose Hue/Saturation. Drag the Saturation slider all the way to the left to remove all the color from the image. Click OK.
Click on the Background layer and then from the adjustment layer popup menu, choose Color Balance. Use the buttons at the bottom of the dialog to determine which range of tones you want to work on – here I started with the shadows. Then drag the color sliders to “mix in” different colors (this is definitely a case of trial and error).
Now click on a different button (highlights in this example) and again move the color sliders. Experiment with changing the color sliders with the midtones button selected.
One of the potential disadvantages of using Channel Mixer – that this method “solves” – is that it’s very easy to blow-out areas of the photo as you move the Channel Mixer sliders. By combining the Hue/Saturation and Color Balance adjustment layers, it’s much easier to get the results you want, without as much worry about introducing blown-out areas (or said a different way, using this method, it’s much harder to blow-out details).