Everyone suffers from compulsive decorating syndrome, i.e. the need to change the way your house looks once a week/month/year. Yet for those of us on a budget, often our only way to satisfy this craving is to rearrange the furniture and buy a new lamp.
Want to totally transform your room and not completely empty your wallet? Consider hanging a pretty curtain on a bare wall that has no windows. This is ten times easier than repainting, and if you get sick of it? Take it down and buy a different curtain! If you've got the wall space, try letting about a foot of the wall near the ceiling show - it will make the room look larger by elongating the wall.
Also make sure that you don't have any furniture directly against the wall/curtain - you don't want ordinary jostling to keep tugging at your pretty new wall.
One place where the curtain will look especially lovely - the bedroom! Try getting a tufted headboard, then put the curtain on the wall behind your bed.
Consider the curtain/headboard combo shown below (from UrbanOutfitters.com):