Some painters start a painting with the highlights, some with the extreme darks, and then make sure these are maintained throughout the painting. It's easier than starting with mid-tones.
When your painting is 'finished', check whether you've still got your "darkest darks" and "lightest lights". If you haven't, the painting isn't finished yet and you need to adjust the tones.
Read more: Adjusting Color and Value, in 7 Steps to a Successful Painting.