Frequency separation is an amazing technique used to retouch skin. It separates the color from the texture and allows you to work on them separately. In this tutorial, Elena Jasic goes over how she uses frequency separation in her retouching workflow. Elena's retouching work is absolutely gorgeous, so when she did this video on how to do frequency separation I was ecstatic, and right after watching it I knew I wanted to share it, including some other advice she gave me.
You will notice that the brush shape she uses for the texture layer looks like a small diagonal line. I asked her if it was a special brush and she said she just modified the normal round brush to be slanted a little. She said she feels like it helps her more with accurate retouching on the texture layers.
Here is a screen shot she sent me of her brush settings. I can tell you that after watching her tutorial and trying it out with this brush, it does seem to make a significant difference in my retouching workflow as well.