Quick Skin Walkthrough

I really can't "teach" how to do skin, since there's so many skin types, but if I can at least give a quick walkthrough, maybe it will be helpful to someone!

First of all, and probably what I think is the most important, is that shadows are not always just GREY or a darker shade of the main colour. Use OTHER colours, red, purple, blue, etc. Same with highlights; yellow, orange, typically warm colours.
If you have a "Brightness/Contrast" setting type thing (I know photoshop has it, since that's what I use), experiment with that, you'd be amazed at what it can do.

I'm no expert, but if you have questions, feel free to ask!