You can make your apartment look bigger, even though you obviously can't add on or tear down walls. About.com has a lot of great suggestions for ways to decorate your apartment so it appears larger than it is.
If your landlord lets you paint, choose light colors for your walls, which will make a room feel bigger. Once you've chosen a color you like, bring home a sample and paint part of one wall so you can get a feel for how it looks in the room. It might look darker in your room than it does in the bright fluorescents of the hardware store. You also should avoid dark colors or wood stains on furniture and curtains to prevent the room from seeming small.
A clutter-filled apartment will also feel much more compact. Ensuring you have a place for everything will make your space feel larger and more open. Also, when you pick items to hang on your walls, consider leaving a lot of white space around them. Walls that are filled to the brim with art and other decorations can overwhelm a room.
Leave space around furnishings and don't crowd the room with extra pieces to make it look and feel bigger. Also, consider pieces that have a smaller scale. A loveseat will fit much better in a diminutive room than an overstuffed sectional.
Finally, consider lighting, air quality and mirrors to visually expand your space. Just opening curtains during the day will have a dramatic effect on your space, and the same with opening windows. Mirrors can also sell the impression that your apartment is bigger than it actually is.