In reference to paintings, I had an instructor years ago who commented that "ít's the greys that make the colors sing." To me this means a number of things. First, the grays (neutrals in negative spaces) give the eye somewhere to rest, and make areas of color pop. This doesn't mean that the negative areas are necesssarily gray, and certainly  doesn't mean they should be boring. It does mean that, unlike our emotions, negative space can be very positive!  I  paint and love color, but I think the concept holds for drawing (and sculpture, etc.)

Just as what someone doesn't say often says more than the words they emit, negative space contributes immensely to the totality of the piece of art. My preference in drawing is that negative space - areas around and the windows in the drawing - are the kind of gray that makes the colorful dance of the pencil sing. Again, I don't think this means gray literally, just that negative space is a purposeful foil - sometimes filled with strokes, sometimes left as the plain paper - to the subject. It should help the subject to pop - it's part of the story, too........what are you saying when you "don't"say anything?

Indeed, Garwood.

I recall reading a quote from Miles Davis in which he said his music is in what he DOESN'T play. Certainly, Miles played silence better than just about anyone. He knew when to play sparsely and went to blow.

Well, this is a different type of answer, but I use negative space as a form of measurement.  For instance, if I am drawing someone's arm, I will look at the space between that person's body (or whatever might be connected to it or nearby) and arm and use the shape of that space, rather than the arm itself, to establish length, shape, value, degree of contrast, and any number of things.  I may use the space to soften or delineate edges or shadows as well.

Reading this was a good reminder to think about negative space. It's just a habit to focus on the subject of the painting -- the portrait, for example -- and forget the importance of the space where the portrait is not. (How's that for a Zen koan?)