Look for the Fall of Light
When working with a single primary light source, you will notice that there tends to be a graduating fall of light across the figure – with the parts of the figure closer to the light source being generally brighter than the forms further away from the light. Even if this isn’t too apparent in your reference or in real life, I recommend emphasising the effect in your drawing, as it lends a sense of unity to the whole figure and will make your drawing more luminous.
It is most evident when a standing figure is lit from above. As the upward facing planes on the top of the shoulder will be much lighter than the upward facing planes on the feet. Despite facing the light source at the exact same angle as the foot, the closer proximity of the shoulder to the light source will make it much lighter.
To ensure that the drawing is luminous, this principle should apply to all the halftones that are the same distance from the light source. So halftones on the shoulder and chest will be generally lighter than equivalent halftones in the lower leg.
Add Darkest Halftone Masses First
Begin by blocking in the darkest halftones as large planes, don’t worry about trying to add gradiated shading at the beginning. It’s best to start with large masses of halftone first and then work within them as you add detail.
Be Careful Near the Lightest Parts of the Drawing!
In order to ensure that the fall of light/luminosity is maintained, I recommend that you are careful to avoid adding tone to the lightest parts of the drawing until all the other half tones have been added. The lightest areas should only need very light and subtle halftones to turn the forms. Adding too much tone to the lightest areas will make your drawing at best over-modelled and at worst like the figure is covered in a dirty grey colour. Even when working with darker skinned figures, this is still important, as the fall of light will still occur even if the tones in the lights are generally darker. However the highlights on darker figures become more important because the difference between the lightest areas and the highlights is more pronounced.