how do you retouch skin that's covered by strands of hair? like shoulders or forehead...

Very carefully! OK, now the real answer: like most techniques in Photoshop, there are several ways to handle this and what method you use would depend on just how much hair you're dealing with.

If you have just a few hairs to work around, you could retouch over the hair completely and then carefully History Brush the hair back (Wacom tablet works wonders for this). If you have more hair to deal with, you could make a Color Range or Quick Mask selection of the hair and bring it to a new layer, which would allow you to retouch over the hair in the background layer. Or, if the hair you're dealing with is less defined, use softer brushes when retouching the skin and carefully fade off the retouching as you get closer to the hair.

Post an example and I bet you'll get a good handful of different approaches. Trial and error is the key . . . each retouch may require different tools and techniques. Hope this helps!

jp