I would try to impart a melancholy feeling on the readers. Thats the best way I can describe my depression to others, and when I see something on TV like "The Perks of being a Wallflower," I would say my overwhelming feeling is melancholy. Perhaps this can be achieved by a lot of first person narrative, reflective thoughts, and something that the readers can identify with even if not depressed. For example, trying to make something of themselves is pretty much a universal struggle.