When I started quilting 20 years ago I couldn't afford quilting books and certainly didn't have any internet access to look at how other people did things. I had read how to miter somewhere that it said to fold the quilt at the corner in a 45 degree angle, draw a line out to the edge of the border pieces, and sew on that line. That didn't always come out right for me and I was dissatisfied with the results. What I wanted was that knife-edge straight line I got when I pressed the miter so I came up with a way to keep that nice edge and alignment.
I don't know if anyone else does this and I haven't seen it anywhere so, here goes with a tutorial on it.
When I am mitering a corner I still measure the quilt through the middle and instead of cutting the border that length (you need more length past the ends for the miter), I use a fabric marking pen and mark the quilt edge line on the border. If your border is 6" wide then you need to have at least 6" past the edge of the quilt. I add at least 1" past the needed length.
Sew the first two opposite sides of the border on as usual. Mark and sew the second two opposite sides the same way and be sure to start and stop the stitching exactly at the edge of the quilt, don't sew into the opposing border at the corners.
Lay the top on a pressing board, making sure it is square, and carefully press the borders.