Skin retouching is a fine skill that takes a good sense of nuance and a careful eye for detail. This helpful video will show you a fairly standard and effective workflow for skin retouching using Photoshop.
Coming to you from Joel Grimes, this great video will show you his skin retouching workflow. Skin retouching is one of the most fundamental skills you need to have down pat as a portrait photographer (or anyone who works closely with people), and it definitely takes a bit of practice to gain an instinct and feel for how much to do. In particular, frequency separation is a technique that has caused a bit of controversy in the industry over the years. I personally don't think it's a bad technique to use; it's just especially easy to go way overboard with it, as it's a very powerful thing. If you've not seen it before; what Grimes is essentially doing is separating the texture of the image from the colors and luminance information so as to work on them separately. In general, especially if you're new to retouching, try to err on the side of doing less. It's always better than overdoing it.