TrustedReviews have a feature about adjusting your colour photos to create
great looking black and white versions.

Although colour photography is far more popular these days, there’s still something very attractive about nice black and white print. However you've probably noticed that if you simply set your camera to black & white, or convert a colour image to greyscale (removing the colour information) using your image editing software, the resulting image looks very flat and uninteresting.

Image editing tutorial - Monochrome from colour