I actually started looking for tutorials on this topic last night. Spending the time tracking the tutorials down was harder than making a decent skin texture after I found them. (And decent is good enough for me- I have never needed to do an extreme close up.)

This tutorial is very old, but is an excellent start-up guide.

This one is a good help with skin tones.

And this was a nice find for a technique to use shaders to give extra texture in rendering. Though I wouldn't follow the instructions in this tutorial exactly, it gave me excellent insight on some more things I could do in the material room.