Clean it and change the furniture round.
No disrespect, but by cleaning carpets, curtains and furniture (if you cannot afford to replace) can work wonders. Not only can it brighten the room, but also makes it smell nice too.
If you can afford to, repainting makes heaps of difference--only try to make a better job than our landlord did.
New cushions don't necessarily have to cost the earth, nor do pictures or other accents like lamps.
A clean change of view can be quite cathartic and can change the way you feel about a room.