Here is a challenge for you :
Brush Economy. No smooth shading. But thinking about every brush stroke you lay down. Not sure how you doing these studies, but when you zoom in on Sargents work, what makes it authentic is the decisions of each brush strokes he lays down.
I noticed, especially for digital painting, we seems to let the tools such as smudging or blending do the work for us, instead of doing the opposite, thinking about the brush marks you put down, thinking about the direction of the stroke etc.