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 a 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 a Color Balance adjustment layer.
Here's the original image I used for this technique.
Click on the Create New Adjustment Layer icon at the bottom of the Layers palette, and from the pop-up 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 Create New Adjustment Layer pop-up menu, choose Color Balance. Use the radio buttons at the bottom of the dialog to determine which range of tones you want to work onhere I started with 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 radio button (Highlights, in this example) and again move the color sliders. Then experiment with changing the color sliders with the Midtones radio button selected, and click OK.
One of the potential disadvantages of using Channel Mixerthat this method solvesis 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).
Here I changed the blend mode of the Color Balance adjustment layer to Luminosity.
Variation 1: Luminosity blend mode
One of the variations that Katrin mentioned in her class was to use a Selective Color adjustment layer with the Hue/Saturation layer. Here I used Layer>Change Layer Content to change the Color Balance layer to a Selective Color layer. Then I experimented with the different choices under the Colors pop-up menu and moved the color sliders in the Selective Color Options dialog.
Variation 2: Selective Color adjustment layer
Here, I wanted to create an image that gradually changed from color to black and white, so I selected the two adjustment layers and used the Layers palette's flyout menu to choose New Group from Layers. Then I added a layer mask to the Group (by clicking on the Add Layer Mask icon at the bottom of the Layers palette) and applied a black-to-white gradient with the Gradient tool (G).