Value is probably the first element of design you learned when you started painting. It's all about the differences between light and dark that not only show your viewer what the light is doing, but also help to define the shapes of objects in your picture, regardless of their colors. It takes practice to be able to recognize correct values. One thing that helps is to map out the values in value sketches prior to starting the painting. Another is to take black-and-white photos or scans of your finished artwork to check and see how your values are working. Shoot a whole roll of different paintings to see how effectively you're using value contrast in your artwork.
For more information on value and color, see Exploring Color Revised pp. 23, 26-27.