This makes a balance of the skin tones that I like a lot.
If the specific image has a lot of green or blue in it you might have to tweak it some, but it's a good base level to start with.
The last thing I do is make a curves adjustment layer and bring up the midtones just a little. Say something like an Input of "75" and and output of "85" to add just a little more punch to it. If you haven't played with the channel mixer before to do black and white conversions, give it a try.