I use machine sewing needles that are no longer fit to stitch as a modified type of thumb tack. They make a very tiny hole in your wall, they don't do damage to your wallhanging -- after all, the needle has been through the layers many times already, they're "free", and they are amazingly strong. I'd say I use one at either side and then maybe one or two additional if the wallhanging is big enough to have some weight that needs distributing. They also pretty much disappear, so you don't see anything unless you are right up there with your face next to the quilt.

For quilts less than 24" wide, I put 2 triangles in the upper corners. Fold a square in half diagonally, stitch and turn right side out. Then hand sew to the upper corners. A dowel will slide into these triangle pockets easily or a section of a wooden ruler/yardstick works too or any flat piece of wood. I suppose a skinny spring curtain rod would fit in there too. If using wood, I just drill a small hole in the center to fit the brad I'll put in the wall. Then push the rod over the brad and you are done. If it's a larger quilt, then I use a sleeve sewn to the top edge. If the quilt has binding on it, put the sleeve under the binding. Then the top edge will get sewn on when you sew the binding on.