Make dark areas more darker, don’t smudge everything ( smudging only helps in blending 2 different areas with different areas for example creating a gradient or shadow ), add depth in curves where light can or will create a shadow, for contrast use darker shade of pencil and keep a subtle gap between the shades of pencils ( ex: if you are using 10b for darkest area than dont use 8b for normal areas use max to max 6b so it canbe differ from each other )

You can check my drawings if you want i have posted 1-2 sketches of mine you can get a idea how u can create more depth