This is a simple version of the technique, but has many variations that give you lots of flexibility in how you decide to "expose" your black and white image. Some variations that you can try:

• Use the screen blending mode. This allows you to use a channel to just lighten an image. Black areas do not darken the layer below.
• Use layer masking. This will let you block out areas of the channel that you don't want to affect the overall image.
• Make levels and curves adjustments to individual channels.

A Quick Caveat

There is much you can do to get the black and white image you want, but consider this caveat. There is a point where you are no longer adjusting the image, but rather significantly manipulating it. Although I do not think this is necessarily wrong, its a good idea when significant adjustments have been made in an image to disclose this. Don't feel bad about doing this. Ansel Adams, a master of the black and white landscape, made an art out of manipulating print exposure in the darkroom. What gives his photos their strength though is not the details of the exposure. The characteristic that gives a photo an emotional impact is the composition. Work on this, and your photos should be all the stronger.