Although I have used mental ray skin shaders before my skill at using them wasn't as good as I'd liked. I've had a more indepth look at them. There are 3 main focuses with the skin shader, epidermal scatter, subdermal scatter and back scatter, all of which need to be adjusted dependent on the size of scene and object. For me the most interesting part to get right is the back scatter. Back radius and back scatter depth determine how far the light will pass through an object, much the same way light will pass through your hand when held up to the light.