Long, Narrow Room
A long, narrow room runs the risk of looking like a never-ending hallway if care isn't taken with furniture arrangement. Lining up furniture flush against the long walls and leaving the short wall empty intensifies the problem. Place furniture in nooks with long sofas or tables placed perpendicular to the long walls. Place a piece of furniture on the short wall to give the eyes somewhere to "stop." A wall hanging behind it works well, too. Create tiny nooks for each activity while keeping paths open for traffic. Use short bookcases, sofas or tables to divide the room.
Some rooms may have one side cut off with a diagonal wall, most often adorned with a bookcase or fireplace. Place furniture in relation to the focal point, placing a sofa facing the diagonal and a love seat at a 90-degree angle to the sofa, for instance. You can also frame the diagonal depending on the room shape. Set a pair of chairs on either side of a bookcase wall, placed at a diagonal toward the center of the room, for instance.
Rooms With Rounded Elements
If you have a room with a bay window or a rounded architectural element, treat that as a focal point and arrange accordingly. While bulky, angular furniture may look awkward lined up against the rounded wall, placing a comfortable armchair, tiny side table and small area rug next to it fills the space but doesn't compete with its shape. A miniature bookcase may work well, too, depending on the wall size. A small office desk may work well, or if it's in your bedroom, use the window instead of a headboard, or turn the bed to the side and dress it as a sofa for a sitting and sleeping area.
A large, rectangular room may seem impersonal if furniture is spaced too far apart, but it can look cluttered if plenty of heavier, bulky furniture is crowded in. Consider what activities will go on in the room. If it's a family room that needs a reading, work and game area, create three separate areas where furniture may sit across or diagonal from one another. Tuck in furniture for a cozy feel but leave enough room for pathways between nooks. Define areas with rugs and clustering art and accent pieces.