I found this great trick on PetaPixel.

Applies to Photoshop CS5 and above.

1. Create a B&W adjustment layer above the background layer**.
2. Set the blending mode of the B&W layer to "Luminosity"
3. Adjust the individual color sliders to suit.

**Your image will change to B&W. When you do step 2, you'll see the original color image.

You can also use the B&W adjustment layer to convert a color image to B&W. In this case, leave the blending mode set to 'Normal'. You can then adjust the brightness of colors as in step 3, above.

--P