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Answer:
1. Start by making a new layer in Photoshop, select Overlay mode and check
the "fill with overlay neutral color" radio button in the dialog.
You can also use Soft Light for a more subtle application of noise or Hard
Light for heavier noise.
2. Apply noise to this neutral gray layer only. In RGB this will be a true
50% gray but in CMYK the channel values depend on the CMYK set-up.
3. Flatten before output
This method has the advantage of ramping the noise off in highlights and
shadows with zero noise in black and white values thus preventing a dirty
look. This also allows you to add more noise that you might be able to
otherwise in the most troublesome areas of banding. You might also experiment
with applying slightly different amounts of noise to individual channels
(still only on your noise layer) with higher percentages in channels with the
most banding.
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