We purchased enough Wonder Clips to go all the way around a quilt. We lay the quilt out on our cutting table and smooth out the outside 8" or so. Then we lay the binding on the edge of the quilt smoothing it but not stretching the edges. We clip the binding to the quilt edge. We go all the way around it arranging the quilt so we have that flat 8" edge to work with. We arrange the corners just as you would stitch them. By doing this we are also able to go back and rearrange the border so none of the joining seams end up in a corner. Haven't had trouble with wavy borders.

Is your quilting evenly spaced across the quilt? If you have areas that are more densely stitched and other areas where the quilting lines are farther apart, this will cause the quilt edges to be wavy.

The quilt's I have hanging in my house have two sleeves sewing in them, one at the top and one at the bottom. I use café curtain rods which only stick out about 1.5 inches from the wall. One is used to hang and the other is simply placed in the bottom to keep down the waves. The weight of the rod helps.