Contrast is tricky; it's generally better to go overkill than try and be subtle. Our brains put together shapes and depths largely from contrast, not color, so when you're emphasizing form, it's usually better to use a little stronger light and dark on the borders and areas that are in occlusion or are going to give us a highlight.
Also bear in mind that I just made a lot of arbitrary decisions about depth there; there were lots of ways to interpret the art as it was, I just made a few choices to illustrate the concepts
I usually do it in black / white with Overlay or Soft Light, then tune down until it quits being effective, then back up a bit. For deep shadow, I select areas and then just airbrush with veeeeeery slooooooow settings for flow, 3-10%.