just a quick mention:

you can't see negatives when they're colors.  but you can when they're affecting something else.

this gets back to what i mentioned about the displacemen map solution.  the point is that you want some of your displacement to go into the figure, not out.  then you need to subtract from the greyscale map the color or value of the color you want to be your state where there's no displacement.  with a color math node, you can actually use your eyedropper to select this color in the little preview of your map.  not a very precise method, but it can come in handy sometimes when you don't need to be precise. 

once you subtract anything greater than 0 from your displacement map, some of it will inset rather than expand. 

and i have to add this:  this is very important if you want to have significant displacement and correct for people using an altered diffuse map for a displacement map instead of something more refined.  because lots of that image will tell the skin to do something you don't want.  most likely expand.  if the values are low, you won't notice the difference, but if for some reason you wanted higher values, you'll puff up your figure.

also, your displacement map can go beyond 1.    so the whiter than white results won't be leveled off.  to level off the results, you should use a clamp node, as described in the Castle Poser tutorial and shown in the Castle Poser example material.