The best place to start decorating your long, narrow room is at the focal point. Do you have a fireplace, or a window with a great view? Built-in cabinetry or a stunning piece of furniture or art? If the room doesn't have a natural focal point, you can create your own. You want to ensure the eye will be drawn to a great feature of the room, which will help distract from the fact that its silhouette closely resembles a hallway. A word of caution: You definitely don't want the focal point to be on the end wall -- this will just accentuate the shape of the room.
Paint is a great way to help shape up an awkward room, too. Cool, light colors make the walls seem like they're receding, so they're best used on the long walls. Warm, dark colors will work best on the shorter walls to make them seem like they're advancing toward you, and this combo will help to visually balance out the room. Also, a linear arrangement of bookshelves on the short wall is a great way to make it look a little wider.
Lighting is another way to trick the eye into thinking the space is less awkwardly shaped. Recessed lighting creates pools of brighter light that help your eye focus on smaller areas of the room, rather than viewing the room as a whole. It can also be directed toward items in the room that you want to draw attention to, such as a collection or a piece of art.