Thank you, Amy, for asking about this! If YOU’re thinking about walls over sofas, chances are other people are, too ;)

First things first, Gentle Readers: Do you have a fireplace? If so, what are you hanging above that? I am a believer in using mirrors sparingly, so if you have a mirror over the fireplace, ix-nay on one large irror-may over the sofa.

If you have art over the fireplace, though, you may consider:
A mirror over the sofa. That’s Option 1. I recommend this grudgingly, actually, because I’m not a fan of using mirrors in place of art. That said, I’m well aware that good, large (remember those TV ads for “sofa-size” paintings?) artwork can be tough to find.

So yes, I’m ok with hanging an interesting (round counts), or at least elegantly framed, mirror over your sofa. Bonus points for hanging a smaller painting on either side of it. Those pieces don’t have to match, but they should be roughly the same size as each other.
(Below, pretend that the Bisa Butler quilt is the mirror and the things on the side are paintings.)

Option 2: One large painting (or a diptych or triptych, which reads as one piece). This is kind of an obnoxious answer, actually, because it’s so obvious, and if you’re asking the question you likely don’t have that magic piece yet. Nonetheless, mention it I must.

Option 3: Three medium-to-large pieces.
On the floor, place the largest piece on the left, and stack the other two on the right. Leave around 4″ between everything. How does that look?

If, after you adjust and tweak, the arrangement is visually satisfying, measure the width of the entire arrangement. The center of that, and not the center of the space between the L and R “columns” goes over the middle of your sofa.
Option 4: A series hung in a grid, e.g. English botanical prints or architectural engravings. The more the merrier.

Option 5: A gallery wall. This easily could become a blog post about gallery walls — and maybe it’s time to do a post about that. But for now, gather 5 or 7 pieces (odd numbers are easiest to work with). And then…there are no rules. I’m sorry. Annoying, right? You just have to lay everything out on the floor or dining table (on a tablecloth to avoid scratches) and move things around.
One thing I can say, though: do NOT to place the largest picture in the center of the arrangement. Make it the left or right anchor instead. Otherwise, it’s all about that large center piece, and the goals is to create a harmonious arrangement in which each item is receiving equal attention.

What do I advise against? Family photographs over the sofa, even if they’re beautiful black-and-white or sepia toned pictures of your ancestors. I always feel as though you can’t get close enough to really SEE the people. And I want to see where your eyebrows came from! Family photographs do really well in hallways, where you’re forced to see them close up and at eye level.
I hope one of these recommendations will help you create a beautiful space in which you and the people (and/or creatures) you love can spend time. Or in which you can spend some glorious, precious time all by yourself.

My new book, My Neighbor Saw Me Naked, and Other Reasons You Need Drapes: The Essential Decorating Handbook, is now available wherever books are sold. I hope to see you at a book event soon!