One of the most sought after retouching tricks is how to clean up the skin without losing the texture and detail, something you may see in make-up and beauty ads everywhere. The technique is pretty time consuming and is really only worthwhile when doing ad work or very high end editorials, But the results make all other retouching just look bad.
I trained one of my interns in this technique and to fortify what she learned I had her make this video of her retouching one of Alex Covo's beauty images.
This will take some practice to get it looking smooth and clean as it will require you to pay close attention to detail.
Just remember not to retouch your retouching.