4-Furniture placement, of which we have a good example in our living room/dining room combo. When sitting at our dining room table you see the back of our sofa. There's an ample walkway between the head chair and the sofa which sends the message that we've transitioned into another room.
One additional thing you can do along those lines are add a sofa table behind the sofa which gives an even cleaner break and creates an opportunity for more accenting.