It's very simple actually. Duplicate the background layer of the colored image - convert only the duplicated color layer to B&W (by desaturating or a better method) and adjsut the gray tones so that you get a good balance of light and dark tones. Then adjust the opacity of the B&W layer so that the colored background layer shines through. I don't remember exactly what the opacity % was but use something like 50/50 to start with and see if you like it.
The same technique was used in this image
http://www.pbase.com/julianht/image/51802916 and several others in my pbase galleries.
Julian