Layering, not pressure

There are two ways to make a middle grey: press hard with a hard pencil, or lay two or three light passes with a soft one. They look similar at arm's length and behave completely differently afterward. The pressed version has grooved the paper, polished the graphite until it shines in the light, and sealed the tooth so that nothing darker will stick to it. The layered version is matte, still has tooth, and can be taken darker or lifted lighter at any time.

So the working rule is: light layers, soft pencil, change direction between layers. The first layer establishes the shape of the tone, the second deepens it, the third is reserved for the darkest part. Three layers with a 2B will take you to a solid value 7. A fourth with a 6B reaches 9.

Every area in a finished drawing should still accept another layer. If a passage has gone shiny, it is finished whether you like it or not.