I got an email last night from someone wanting to know how I get perfect skin color in every retouch...the short answer is that they aren't perfect. Not by a long shot. I do try to get something that's pleasing at the very least.
No two people have the exact same skin tone...so much depends on race, environment etc. I have a set of tone which I've blended and converted into Photoshop swatches. When a subjects skin needs a little help I rely on those as a "filler".
I really only use the swatches in extreme cases where a person's skin is so uneven that none of my usual retouching tricks get's me to the picture in my head. I first get the skin cleaned up as best I can correcting exposure and white balance then fixing blemishes. That get's me in the ballpark so I can spot areas that will benefit from "painting".
My preferred method
1) Create a blank layer and select the "Color" blend mode
2) Set the layer Opacity anywhere between 5 - 10%
3)Select a soft brush and set it to 20 - 30% Opacity
4) Select an area of the subjects skin that you consider to be ideal
In my case, I select a tone from my Skin Tone Swatch.
5) Brush the color into the areas that you feel needs the boost. Take your time to avoid features like the lips and eyes...unless you want to target those areas.
6) Apply a blur filter...I use about 40% to smooth out any hard transition areas on that color layer.
From there I tweak it in with Dodge & Burn layers until the image looks the way I pictured it.