I lay in the shadow family first
The more I paint, the more I am convinced that establishing an effective value pattern is the most important aspect of a successful picture. This can be quite a challenge when painting out of doors. The past several sessions of outdoor painting with my students has been evidence of this. I've been reading a lot these days about plein air painting including Carlson's Guide to Landscape Painting as well as from Fill Your Oil Paintings With Light And Color by Kevin D Macpherson.
In the book Macpherson advocates simplifying the value pattern into to "families" consisting of the light family and the shadow family. The light family is anything that the light source illuminates and the shadow family contains areas of the composition that are not directly illuminated. In order to create a clear matrix or value pattern, keep the two groups distinct and separate from one another. You can do this by remembering that the lightest area of the shadow family wants to remain darker than the darkest area of the light family.
By following this rule, you will not intermingle your value pattern with confusing lights and darks and your masses will be distinct. Of course this can be a challenge when the light is overcast. In this situation, you will rely solely on the local values of each mass within the composition and plan your value arrangement accordingly. Our painting session ended up more overcast than sunny so my light and shadow families are not as distinct, but the light and dark value pattern persists and the painting works for the most part because of this. I spent about an hour and 40 minutes on this study.