Adjustment layers are almost mandatory for non-destructive and efficient work. They greatly reduce file size instead of copying a layer and making adjustments, and are very useful for painting in shadows and light. To do this, I will often grab the Rectangular Marquee Tool to select a box over the area I want to adjust. Then, I will create a Curves adjustment layer which will automatically put that box in the mask. I then proceed to lower/raise the values in RGB, and then go into the other colours and either add cool values for shadows or warm values for light. Once I feel like it matches or fits the desired look, I then delete the mask and paint in what I need with a brush.