i think a lot of knowledge about retouching does not come from photoshop-skills, but by observation. starting with your local nude drawing classes to spectating the pros.
well here are some websites, that do not just pose their work, but show the whole process:
Detouch
this site is just awesome. those guys collected before-and-after material from several sites (the links on the right are sets through which you can flip with the buttons at the bottom) and let processing spit out some super interesting behind the scenes material, where you can see each altered pixel and such (information behind the process)
btw - don't miss out the first picture of the fluideffect.com-Set - this one's just hilarious :)
Taylor James
Super inspiring team. and the step-by-step making-ofs are just huge and impressive
Glenn Feron
actually - i never heard of him, but the before-after function is always a nice bonus
Christophe Huet
definitely one of my favourite artists ever. he's not so much into portrait-retouch, but his photoshop-skills are just killer!
Brian Dilg
Greg Apodaca
only a few...but still...
i'll add some more in time, but this is alreadyenough to look at :)
(for all you thirsty eyes: Link)
(truth be told, the spoiling of our view on human perfection scares me. lots! dove made a huge video concerning this topic, i hope you all know (even tho dove are no so moral company as this video shall show...but well...it's still advertising...enlighten yourselves)
