Don't obstruct a doorway with furniture. In particular, the back of a sofa facing a door is like a red light stopping traffic. It blocks entry to the room, or at least forces people to walk all the way around the long obstruction to get into the room. Instead, place the sofa against a wall or with one of its sides facing the door. That way, people can walk either in front of it or behind it when entering the room.
Place functional groups of furniture where traffic patterns don't lead through them. A functional group is a set of pieces that go together, such as a group of seating pieces for a conversation area, an easy chair and small table for a reading area, or seating pieces and the TV for a TV area. Cluster these groups of furniture outside of traffic routes if at all possible, so conversation, work or TV viewing aren't interrupted every time someone comes into the room.
Use area rugs to indicate visually which parts of the room are used for different purposes. A rug in front of the couch with a coffee table on it and chairs to either side indicates the conversation area, for example. The rugs send a subtle message that people should walk around them to get through the room.
Provide easy access to the seating in a small room where it's not possible to route traffic around a conversation, TV or work area. If you can't avoid interruptions, at least make it easy to get comfortable.