For me, the first step to creating a nice wall behind a desk or couch is to find all those little paintings and objects that you have but have never hung them up (you know you have them!) Then, lay them out on the floor using your creative eye to see what looks best.
Start hanging and then add to them as you start seeing other pieces that you find. This way, you have things that you already love and you don't have to go out searching for newbies. I did that in this room and it's now become one of my favorite rooms in the house.