In the top photograph I have changed the image to monochrome in photoshop. I then  imported a different sky as the original was bleached out, the chosen sky matched the mood I wanted to create in the image. The easiest method of several I use is to import it on a separate layer and change the blending mode to darken. This allows all pixels of a darker sky to show through the bleached out sky.

Bottom photographs are both conversions from colour digital files. I prefer to use an adjustment channel mixer layer, which gives me the ability to change the blue, red and green filters to achieve the full tonal range I am after.

Pentax *ist D Pentax 28 - 135mm lens.