“Orient your shading lines such that they show the path that a raindrop on the rock would take across the surface (down) and horizontal for a flat surface. If the plane is not facing straight toward the viewer, down will not be vertical but at an angle.”
— John Muir Laws, How to Draw Rocks
Another one I know, though I can’t remember from where, is it visualise how your fingers curve if you pick up an object, and make your lines/brushmarks follow that.
