Magic!

Well, dear quilters, blogger isn't behaving, so forgive the formatting. I'm demonstrating how to glue for a perfect miter. Start out like you would any miter, mark a quarter inch from the corners of your tops and sew to that point exactly (I like to shrink my stitch length). Make sure you backstitch or lockstitch so it's secure. Once you've done that, lay the corner to be mitered on your ironing board like shown below.



Make sure the side piece is really going straight up. Then fold the top piece back at a 45 degree angle, matching your points if you've got multiple borders. Give it a healthy shot of Best Press and iron by setting: do not rub your iron back and forth, you'll distort the corner.
Peel your nice crisp fold back and run a very thin line of Elmer's School Glue (I prefer the gel, but any ol' elmer's SCHOOL glue will work--not Glue-All, be careful, they look similar).
Next lay it back where it was, careful to realign your fold, but also make sure that the top strip lines up perfectly with the bottom strip (this ensures a nice 45 degree angle). Iron. Yes, heat set the glue. Again, don't rub it around, just set. Once the glue is dry, it will hold really well. Take it to your iron fold your quilt so the outside is going up in a 45 degree angle and sew from your border attaching stitching right off the edge. Use that great crease as your line.
You should end up with stitching that looks like the stitching above (I've already trimmed to a quarter inch above.) Trim to a quarter inch past your stitching and then press your miter to either side.
Voila! A beautiful foolproof miter. Happy quilting!