I looked around quite a bit for any good tutorials explaining a method for smoothing and retouching skin, and by far the most powerful method I've found is using the
Wavelet Decompose plugin.
It basically let's you decompose your image into different frequencies, and to operate on features isolated in those frequencies. This means you can smooth blotchy skin tones without affecting finer details that keep it looking more realistic (like pores/wrinkles). You can also do the reverse, or all at once, but with varying levels of smoothing you get an amazing level of control over the results.
So since I didn't find a great tutorial outlining a good method or procedure, I went ahead and wrote up a tutorial on how I approach it. Hopefully it can be useful to others in some way! You can find it here:
Getting Around in GIMP - Skin Retouching (Wavelet Decompose)Some examples: