So I have known for a while that the Blue channel is not particularly flattering on skin tones, and tends to show a lot of imperfections when retouching.
This can be used to our advantage by using the Blue channel as a 'guide' for seeing easily areas where some retouching/healing might be needed.
Photoshop guru Calvin Hollywood uses this fact for something he calls a "check layer", where he uses a Ps adjustment layer to desaturate the image, and to reduce the Red channel, while increasing the Blue channel. This shows off skin imperfections nicely, and will update in real time as he heals those imperfections.
GIMP doesn't have adjustment layers, but after playing around for a bit I found out that it can be emulated using solid layers with different layer blending modes.
I detail the steps to reproduce the "check layers" in GIMP on my blog:
The basic idea is to use layers to isolate the blue channel in grayscale on the canvas. Then to heal/retouch areas on the base (color) image. Changes will then be reflected in real time on the canvas.
This script creates the necessary layers above your image for you automatically. When healing, work on the original color layer, and the changes will show as you work.
Once saved in the proper location, the script can be run from:
Filters -> Generic -> Skin Check Layer...
[EDIT] A reader on my blog pointed out that the same effect from subtracting both the red and green can be accomplished with a single layer that subtracts the complement of blue - Yellow. So I've updated the script to just use one subtraction layer.