Just a few tricks I’ve learnt, discovered or regularly use on my own shots.

Burning and dodging

Create a new, transparent layer. Sent blend mode to Overlay. Paint in black where you want to burn the underlying layers and white for dodging. Use shades of gray for lesser degrees (where neutral gray - 127/127/127 - has no effect). Vary layer transparency to adjust overall effect. To increase the effect, duplicate the layer or try a different blending mode (e.g. Multiply or Soft grain). (Taken from John Beardsworth’s excellent Digital B&W Photography book.)

Edge burn

As above but set blend mode to Multiply. Draw a rough selection border around, but not close to, the image edge and invert it. Feather the selection by a few hundred pixels. Bucket-fill the selection with black. Adjust layer transparency as required.

Sharpening

Download the smart-sharpening script from the GIMP FAQ site. Apply as the final step. Edit the sharpening layer mask to remove the effect from areas you don’t want sharpened, such as grainy skies (paint on black). If resizing, resharpen the sharpening layer slightly afterwards using Unsharp Mask (1.0/0.5/0). To avoid sharpening grain, run the Despeckle filter over the mask.

Soft focus glow

Duplicate the image layer. Set blend mode to Multiply. Apply Gaussian Blur (10-30 pixels radius) to the layer. Increase the lower output level for the layer to 10-30 AND/OR add a layer mask containing the same image to avoid darkening the shadows. Increase overall brightness using Curves if required. Reduce layer transparency to avoid too extreme an effect. Excellent for child portraits and mid-tones.

Blown highlights

For small areas, use the Clone tool with a soft brush of approximately half the same size of the area to cover, and set the mode to Value. Clone from a nearby area that has some abstract detail (e.g. leaves in trees).
For larger areas, try selecting using the Contiguous selector with a suitable threshold, then lower the top anchor in Curves. NB. This rarely works well; it’s almost impossible to fix large blown areas convincingly. Reshoot if you can.

Toning

The GIMP Guru has a nice technique using Sample Colourise, but a quick way to warm an image uses Curves. Select a shadow, mid tone or highlight point and fix the other two tonal areas. Move the selected point for each channel by the following amounts vertically: Red +3, Green -3, Blue -12. Save the curve for reuse. Swap these figures around to cool the image (-12, -3, 3). (Numbers from Paul Butzi.)

Improving contrast

Duplicate the layer. Change the blend mode for the new layer to Grain Merge. Reduce opacity to 10-40%. Use a layer mask to mask out either the subject or the surroundings, which helps to differentiate them.

Edits have no effect or image is corrupted

Is there a selection in effect? Check View -> Selections in the menu. Are you applying a colour transformation to a greyscale file? Change Image -> Mode to RGB.

Editing 16 bit images

Download and build the latest version of Cinepaint. Make the initial curves adjustment in this (warning: Cinepaint’s Curves tool is still based on the GIMP 1.x), convert to 8 bit and save to a new file. Make the remaining edits to this file in the GIMP. (I only do this when I need to make severe adjustments that would otherwise cause posterisation on an 8 bit file.)

Editing images at work

If you have spare moments in the office, try the Windows version of the GIMP; it works identically.