Image courtesy of (and copyright) Kelly Ealy (mouse over for original).
I have looked around quite a bit for a good method in GIMP to smooth skin for portrait work. I did find a few tutorials, but they all were mainly concerned with using Gaussian blur with a layer mask to "smooth" the skin. The results were almost always less than satisfactory.
Then I found the Wavelet Decompose plugin, and the search was mostly over for me.
I thought it might be helpful to walk through the theory as I approach it for skin retouching/smoothing, and how Wavelet Decompose fits into my workflow.