You can only let in so much sunlight, or have so many lamps in a small dorm. But you can help bounce that light around the room by hanging a simple mirror. Designers love to use reflective surfaces in general to help brighten up a room, which in turn makes it seem bigger. Mirrors are obviously one of the most effective ways to achieve this effect.
But in addition to maximizing the light, a mirror doubles an image and makes it seem as though a room goes on. This is especially effective with floor-to-ceiling mirrors that at first glance seem like a doorway to another part of the room. This might not be possible in a dorm room, but any mirror will help, and the bigger, the better.
If you can, try placing a large single mirror on the wall opposite a window. This reflects the sunlight best, but can have the added effect of making it feel like you have another window in the dorm room.