We often pass over halls in favor of decorating rooms, seeing them as a utilitarian part of the home: get me from here to there, hallway! But a nicely decorated hallway, like a great pair of boots, is the finishing touch that pulls an entire home (or outfit) together and really makes it sing.
paint is the cheapest way to update a space with the biggest impact. This is true from the largest ballroom down to the narrowest hallway. Paint the hall a light color to help it feel wider and brighter, especially if you lack natural light.
A lot of people favor a nice thick carved molding in a crisp white to help emphasize the architectural elements of the doorways (and windows), but whatever style and color of trim you prefer will have the same effect.