What I do: 1) In RGB space, adjust curves/levels/brightness/contrast to get a good looking color image. 2) Use the channel mixer. [Actually, what I usually do is just look at the separarate Red, Green, and Blue channels, and pick one. This simulates using the B&W filters on the camera to alter image tones.] For precise work, or something in between, use the Channel Mixer. 3) [Image should now be grayscale.] Maybe adjust brightness/contrast again. Convert to duotone, tritone, etc., and try loading different presets until tones look pleasing. 4) Convert to RPG before saving as jpeg.