Highly reflective materials to enclose your plants with. Aluminized Mylar, aluminum (sheet, foil, etc), or matte white paint(you want titanium dioxide as the pigment, that's TiO2) if you can't get anything better.
I've seen a guy grow a pretty decent plant with a couple CFLs and a trash can lined with aluminum foil. I gave him rep for his ingenuity.. should of made a post, because I lost track of it. I've thought many times of a conical grow box, a trash can, how obvious!
Also in canopy/side lighting. Surround bulbs with plants, get my drift? Why use reflectors unnecessarily... why not just grow more plants(or form dense canopies that don't let light through, maximizing absorption).
