1) Open your digital image in Adobe Photoshop.
2) With your file open, go to the layers menu and click the half black and half white circle at the bottom of the window. (If your window is not open, go to Windows>Layers) Now go to Channel Mixer.
3) In the Channel Mixer dialog box, tick the Monochrome box. In the Source Levels section, set Red - 40%, Green - 60%, and Blue -10%. Press OK.
4) Your image should now be Black and White. If it is, you are on the right track. Now get the Eraser and set it to whichever size will allow you to erase the parts of your image that you would like in color.
Just click on the parts that should be in color and you will see how this works wonders by changing it back to color. Erase what you need to stay in color and the remainder of the image will stay black and white.
Just save the image as you please and you're done! Heres my final effect:
