One of the techniques I use is a transparent "wash over". It can be done as a oil/medium wash and then paint right into the wet, or as in this one above, a turpentine wash with some ultramarine blue added and then let it dry.
The goal is, first to drop the value down a step or two. Second, and really the most important, to create a color edge that will stay with the painting and harmonize all the tones, in all the areas.
The scary part is that I can, for a while, completely hide (or ruin) the entire painting and I just have to trust myself that I can get it back.
In the last few years, I have also been working in acrylic, it's just so easy to wash over, wait 20 minutes and start painting again, but oil washing this way means that I do it at the end of the day and go home. UGH!
The reason I did it this time was to get a "depth of darkness" in the forest.
