This tip has been contributed by Maurice F. Hammon from the UK. Maurice says he got it from someone, who got it from someone else who... so we don't know who had the idea in the first place, otherwise we'd give credit... Maybe...

This is a contrast management technique – known, simply, as “50% gray”. It has the advantage of allowing the user to approach any contrast problem selectively by handpicking the areas that need attention, instead of using a one-step, global approach to the whole picture.

First, create a new, empty layer filled with 50% gray with mode set to "Soft Light", opacity to 50% and fill to 100%. You can do this in one step using the Layer>New>Layer command. When you select Soft Light mode, the check box to fill with 50% gray becomes active:

(Note, I changed the default layer name to "50% gray".)