First off I work up a detailed rough in pencil as normal and then scan it in. Once I have the image in Photoshop I turn up the levels and ghost the pencil layer. Ghost is a filter effect I found on flaming pear and it has been so useful over the years that I don't know how I managed without it. The Ghost filter means I can paint underneath the pencil image as if it is on a piece of tracing paper. This makes it incredibly easy to fill the image with a mid grey and then add highlights and shadows just like a charcoal drawing on toned paper and as an extra bonus the original line-work is always available on a separate layer.
The end result is so useful when it comes to keeping control of values on the final painting. However I can't take all the credit because I got the idea for this process after viewing this post by Dan Dos Santos.