What I tend to do when working with a photo is ask if the color serves some functional purpose for that image. If not, I usually convert it to black & white. (If the photo stays in color, I’ll often push the saturation to make it quite vivid—if the composition is about color, I usually like there to be a lot of it.)

Of course, the right way to do black & white digital conversion is not to desaturate the image—the result is usually pretty uninteresting. There are a number of options, including (in Photoshop CS3) a black and white conversion option. Lately I’ve been working with the conversion tools in Camera Raw, which allow value adjustments based on specific colors in the image. That means, for example, that you can make the greens darker or lighter. I always experiment with a range of settings for each color. I also usually apply a color split in Camera Raw with B&W images. That means you can select a color for the lights and another for the darks (and you can adjust exactly where the split between light and dark lies). The image is not strictly B&W, but if you don’t go overboard you can achieve some subtle effects. I usually make the lights a warm color and the darks a cool color, but not always.