I used almost exclusively soft brushes and chalk brushes here, except for the initial lay-in. Creating firm or hard edges is a matter of using a smaller brush at high opacity. Though I'm sure it would look good to use hard-edged, opaque brushes in places - it's the way I see a lot of concept art being done.
I also like working from a rough drawing - not meticulously measured off, just loose, and modifying it as necessary in the early stages. These techniques combine to make it look more like my own interpretation rather than a precise copy of a photo.
This reminds me of Jeff Jones and Brad Holland - a good combination.