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.