1. Size up the situation

If you're living in a small home or apartment, a mammoth couch is a bad idea (it might not even fit through the door!). Think 70 or so inches in length. On the other hand, if you've got a large living room, or tall members of your family who want to lounge in style, 90-plus inches is a must. You might even consider a sectional, which is a couch in two to four sections that fit together and make an "L" or "U" shape. You'll also want to think about the height of the back, the depth of the sofa, and how high it is off the floor. Some numbers to know: back heights are usually 32 to 36 inches; seat heights are generally 18 to 20 inches; and seat depth is around 22 to 24 inches. Think about your needs and choose accordingly. Custom manufacturers can make each sofa to your exact specifications, from fabric and legs, to length and back height.

2. Comfort is key

Just as with mattresses, a person's sofa firmness preference is highly personal. Some folks want to sink into a cushy, plush sofa and lean way back. Others want a more firm, structured piece of furniture. This is important information to know before you decide on a sofa.

3. Consider construction

Durable, high-quality fabric is a must, but what's underneath that pretty exterior explains much of a sofa's longevity and cost. The best sofas have an interior frame made of kiln-dried wood, securely held together with double-doweled joints. Look for eight-way hand-tied coil springs, three or four rows deep, with bands of jute webbing, which make sagging over time less likely. These sofas will be covered with many layers of high-quality foam or down padding, and will have sturdy fabric upholstery. Read up on a manufacturer before you buy—the website should explain how their sofas are constructed. Another option is to work with a professional interior designer, who will know how to source the best sofa you can afford.