The Photoshop experts behind PhotoshopRoadmap.com have compiled a huge list of Photoshop tutorials about the digital enhancement of human faces and bodies. Changing eye color, extracting hair, wrinkle removal, … it’s all there.
So, if you want to learn the basics of photo retouching, this is an excellent start. I would even say: a must-read for every Photoshop afficionado! Read it here.