make a duplicate layer on top of the original pic - then make the duplicate semi-transparent. you can work on either the layer to make it look prettier, but keep the original visible behind it to keep the original characteristics.

you can smooth the skin on the layer with a little blurring, adjust color or brightness, or whatever you like - and the bottom pic will be visible through the layer.

you might have to experiment a little with the opaqueness, but once you get the hang of it, you'll have great results.