i could be wrong, but i shoot everything in RAW, and then change it to black and white with a B&W adjustment layer in PS. This allows for independent adjustment of each color to its respective shade of grey. Theres a bunch of different ways to do it, but i would stay away from just desaturating it...this tends to flatten everything out and destroy contrast.
I agree, here is how I do it:
1. Open your image
2. Create 3 duplicate layers, name them "Red", "Blue", "Green"
3. Make sure the layers are in this order top to bottom: Red, blue, green, original
4. PP each layer like it is a final edited image by itself
5. Turn off all layers except Green
6. Image>Adjustments>Channel Mixer
7. Select Monochrome at the bottom of the Channel Mixer window
8. Make the following values: Red=0, Green=100, Blue=0
9. Hit OK
10. Turn off all layers except Blue
11. Image>Adjustments>Channel Mixer
12. Select Monochrome at the bottom of the Channel Mixer window
13. Make the following values: Red=0, Green=0, Blue=100
14. Hit OK
15. Change layer style of "Blue" to LIGHTEN
16. Turn off all layers except Red
17. Image>Adjustments>Channel Mixer
18. Select Monochrome at the bottom of the Channel Mixer window
19. Make the following values: Red=100, Green=0, Blue=0
20. Hit OK
20a. Change layer style of red to DARKEN
21. Turn on layers Red, Blue, Green (keep original off)
22. LAYER>MERGE VISIBLE (do not flatten)
23. You can now delete the original layer
24. PP your final B&W...and done.
EDIT: I use PS7, so your method may be different depending on your software.