Digital BW, The Print

The best technique I've found to darken skies and reducing grain at the
same time is to use the Burn tool to manually burn in the areas I want
darker.

Choose the Burn tool with really large brush size (I often use sizes as
large as of 500-1500), and use caps-lock so you can see the brush
outline. Use it at opacities of 10-30% and "Range: Midtones". Don't
worry too much about darkening clouds and such too much, you can bring
them back later on by using the Dodge tool with the highlight setting.
Remember that "Range: Midtones" retains shadow detail, while "Range:
Shadows" quickly blocks them up.

I find that this will actually reduce the grain significantly. I think
it was Nick Brandt who posted this technique here a while ago.

I use a pressure sensitive Wacom tablet, that makes any kind of brush
work in Photoshop much easier. With a mouse you will have to be more
careful with the opacities and brush sizes you use.

--
Daniel Staver
http://daniel.staver.no