This tutorial is written following the GIMP 2.4 menus. You can adapt it easily for different configurations.

First of all, if your version of GIMP is in a different language and you want to use GIMP in English to follow this tutorial more easily, here is what you can do, from the GIMP site:

Linux

In LINUX: in console mode, type LANGUAGE=en gimp or LANG=en gimp replacing en by fr, de, ... according to the language you want. Background: By using LANGUAGE=en you're setting an environment variable for the executed program gimp here.

Windows XP

Control Panel → System → Advanced → Environment button in “System Variables” area: Add button: Enter LANG for Name and en for Value. Watch out! You have to click on three successive OK to validate your choice.

If you often change language, you can create a batch file. Open NotePad. Type the following commands :

Save this file as GIMP-EN.BAT (or another name, but always with a .BAT extension. Create shortcut and drag it to your desktop.

Windows ME

Start/Programs/ Accessories/System Tools/System Informations/Tools/System Configuration Utility/"Environment" tab/"New" button: Enter LANG for Name and en for Value.

Windows 95/Windows 98

Under Window 95 and Windows 98, add the line set lang=en in the “C:\autoexec.bat” file.

Apple Mac OS X

Go to System Preferences, click on the International icon, and in the Language tab, the desired language should be the first in the list.

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Now the step of the procedure:

1. open your file
2. change to LAB mode: menu Colors --> Components --> Decompose; in the dialog choose LAB in the Color mode drop-down and tick the Decompose to layers [OK]
3. the LAB decomposed image opens in a new window; open the Layers dialog: menu Dialogs --> Layers
4. (optional) select the L layer and apply an Unsharp mask: menu Filters --> Enhance --> Unsharp mask (as a first attempt choose radius=5, amount 0.5 and threshold=1)
5. (optional) select the L layer and apply a mild S-curve: menu Colors --> Curves and draw a mildly S shaped curve (or press Load and apply a preset curve: you can download good curves for Gimp/Photoshop here: http://fotogenetic.dearingfilm.com/downloads.html).
6. (optional) select the A layer and duplicate it (menu Layer --> Duplicate layer), select the L layer and change its mode to Overlay from the drop-down in the Layers dialog) and merge down (menu Layer --> Merge down)
7. select the A layer and duplicate it (menu Layer --> Duplicate layer), select the A copy #1 layer and change its mode to Overlay from the drop-down in the Layers dialog) and merge down (menu Layer --> Merge down)
8. select the B layer and duplicate it (menu Layer --> Duplicate layer), select the B copy layer and change its mode to Overlay from the drop-down in the Layers dialog) and merge down (menu Layer --> Merge down)
9. recompose the color components: menu Colors --> Components --> Compose and in the dialog choose LAB color mode and verify that there is the right correspondence between layers and components
10. a new image window opens; now you can play with it or you can cut it, paste it in the original image as a new layer and mix the two using the Opacity slider in the Layers dialog after selecting the upper layer

This is a simple and straightforward application of the LAB color mode, but you can play with the optional steps and their parameters and with the opacity of the duplicated overlayed layers.

Googling around you can find lots of others applications (I will study them and report), from grayscaling to selective sharpening.