you should never let your camera do the conversion to bw. you want to do it in a program such as photoshop. and you certainly don't want to simply convert to greyscale or do a desaturate. the reason being is lack of control over the tonalities.
there are dozens of ways to achieve the same result in photoshop, for different things, not only bw conversion. however, i like to choose the process which gives the greatest control.
create an adjustment layer over your image with channel mixer. that's layer>adjustment layer>channel mixer
when the channel mixer window pops up, check on the monochrome box along the bottom. this will convert your image to monochrome with the red value at 100%. now adjust the red, green, and blue sliders to achieve the tones you want. make sure that the percentages all equal out to 100. so you could have red at 33, green at 43, and blue at 30. but they all add up to 100. as you will see, you have great control over the tones, since using each slider individually will be like using a red filter or a blue filter on your lens when shooting bw film. see what happens when you place the green at 100, or just the blue at 100.
here is the original image:
100% red
100% green
100% blue
and the final product, red 60, green 20, blue 20:
there is a more complex way to get the most control out of the bw conversion, but this way is the best balance between simplicity and control.
sean