Whatever method you chose, try and blend as much of the original skin in as possible! I usually try and clean up the original first, by removing as many blemishes as possible with the spot removal tool or similar, and lighting any shadows by dodging and burning.

Then I mask the important areas like the nose, eyes, mouth so they are not affected, and using a duplicate of the original face layer apply a combination of median (to remove the worst imperfections while retaining detail) and gaussian blur (to finish off and hide the subtle edges that median leaves) giving a perfectly smooth result.

Then I add a small amount of monochromatic noise to replicate skin pores and then, importantly, set the opacity of the perfectly smooth layer to no higher than 50%. This ensures that the image does not take on a scary plastic look and retains most of the original character, moles etc. but at a reduced level, in order to get a polished but still very human look.

edit here some examples I've managed to dig out, these are crops of larger images. The first panel is the original, the second is the smoothed layer and the final image is the original plus smoothed layer at 40-45% opacity, I've also included a screenshot showing the mask for the first portrait: