From Phil Metzger’s Drawing Board column in the March 2010 issue of The Artist’s Magazine:
When using flat strokes, do the following:
• Rub the pencil lead briefly against scrap paper after each sharpening to remove any coarse grains of graphite that may cling to the lead as a result of the grinding of the sharpener.
• Use light pressure to avoid dark edges to your strokes. Gently increase pressure to get a gradual darkening of the area you’re working on. This takes some practice.