The BEST method, in my opinion, is to use Image - Adjust - Channel Mixer.  (See bottom of the page if you have PS CS3)

1- Click on for larger image. Click on arrows for even larger image (lower right hand corner). This is the original color shot. It is the Ghost Town of Gilman, Colorado. Use back browser arrow to return to this page after you have viewed the larger version.

2- Now go to Image - Adjust - Channel Mixer.  Notice that the Red Channel is 100% while Blue and Green are 0%. All three channels must add up to 100% (at least for now). In order to see this photo in black and white you must check the Monochrome box (see curser arrow)Photoshop Tips.

3- OK, now lets change the tonal emphasis - sort of like using color filters with black and white film. In this case we are making the green plus 40% - or lighter. To balance the total to 100% I am reducing the blue channel 40% - or darker. Notice that the green grass and trees are lighter and the blue sky is darker.

4- Now let's try something different. Let's make the blue sky lighter (-40%) and the green darker (+40%). Notice the difference? Of course you have a great deal of control using this method. Experiment and be amazed.

5- Our last example is to make both the green channel darker (-35%) and the blue channel darker (-35%) and to bring the total to 100% by making the red channel lighter (+170%). Remember the red channel started at 100%. Now this is a pretty dramatic change in tonal values.  Anyway, you get the idea. Now you can make your black and white images resemble those taken with film. In fact, you can even emulate different films, like infared film. But that's another lesson.