What graphite shading exercises improve layering, values, and portrait texture

[AriaH]

I've recently gotten back into drawing after many years and I'm focusing on graphite pencil techniques to improve my realism, particularly in portraiture. I can achieve a decent likeness in my sketches, but my shading often looks flat and muddy because I struggle with creating a full range of values and smooth transitions. For artists skilled in pencil drawing, what exercises would you recommend for developing better control over pressure and gradation? How do you approach building up layers to achieve deep, rich darks without losing the paper's tooth or making the surface too shiny, and what are your preferred methods for rendering subtle textures like skin pores or fine hair?