It can be very useful to assign numbers to even steps value, often referred to as a value scale. I learned to use an eleven-step value scale- 0-10, with zero being black and 10 being white. The number of steps doesn't really matter, nor does which end is white (some people have 0 as white). But, it helps to pick something and stick with it, since you can then build a library of values to expect for different subjects in different lighting situations.
For example, when I go to paint a model under an artificial fluorescent light, I can expect that a lighter skinned model might have something close to a 3 for the shadow value, going up to an 8. For a darker skinned model the shadow might be closer to 1 or 2, with the main lights around 6 or 7 and highlights approaching 8 or 9. These values might change depending on the exact setup. For example, a light source within the scene might force the light values on the figure to compress down further to leave room for the light source to appear relatively very light in comparison.
It's also useful for keeping values consistent and properly differentiating values. For example, under most circumstances the shadow area will not be as light as the areas in light- even most reflected lights can't reach as high a value. So once you've determined the darkest number in the light, you can make sure to always use a darker number for everything in shadow.
The concept is bit difficult to get used to in transparent mediums such as graphite or watercolor because it's difficult to know when you're at a 5 versus a 6. However, in oil paint it's easy to pre-mix colors in equal steps. You can eyeball it, or mix it to match a predetermined grayscale- either from a good inkjet printer, or from a standard grayscale such as the one determined by Munsell System.
Once you're used to the idea in oil, it can be translated to a transparent medium, although it generally is not as exact- often the darkest dark in a drawing changes slightly as a drawing progresses, and thus it's tough to determine what your 0 value is and how the others should relate. As such, there's a period at the beginning where the value scale is a bit nebulous. It can be useful to draw a quick scale on a small piece of the same paper your drawing is on and compare the values in your drawing to this scale to determine the value numbers. Or, you can keep things more fluid and let the scale evolve naturally. Either way, it's useful to decide which pencils are associated with each value- generally you want to leave the harder pencils for the lighter values, and add softer pencils to the darker values (though you can still use the harder pencils over the soft to smooth things out).
It's worth noting that you don't have to be deadly precise with this for it to be useful, and you don't always need to stick to whole numbers. In some paintings it becomes clear that a value must be between two steps, and in some lighting situations the darks or lights might be so compressed that you can't use the whole steps.