I tend to work from back to front, and maintaining the right value range is one of the more challenging apsects. It is important for me to visualise the whole painting, even though I am only working on one small part of it. At the start this is even trickier as there is nothing else on the page against which to judge the values I am laying down.
I have developed techniques to assist me; the simplest is to first lay down paint in the areas of the lighest and darkest values. This then provides the ends of the spectrum I am working within. Additionally I usually create a loose digital value study of the line sketch, this helps reinforce the mental map I already have, besides aiding with the lighting scheme.
Fightback! - The Value Study.
Painting digitally one has the useful tool of being able to switch modes from full colour to greyscale, this gives a quick and simple check of the values. Clearly this tool isn't available to the traditional painter, but I have recently starting using a tool which approximates that approach: Artgizmos Selectatone Tonal Spectacles.
The spectacles are fitted with a red filter and by donning them it lets you look at a work in progress as a monchrome image. I have found this a very useful addition to the arsenal of approaches for helping determine the value range, and it is easier than standing at the other side of the room whilst squinting.