Next time, sew on the borders and stop at the 1/4" mark from the edge. Iron the rest of the seam allowance. Let one border lie flat and fold the other border until you have the perfect miter. Press that seam. Insert a narrow piece of fusible web (1/4" or less wide) under that top fabric where the seam will be. Iron to fuse the perfect miter into place. Turn over and trim the excess fabric away. Sew on the press mark.
Sandi Irish taught us that method in a presentation on Saturday. She says it works every time!
Edit: Just remember that each border must extend a *lot* beyond where a regular border would stop. You need that extra fabric to create the miter! You want to add to *each* side the width of the border to be mitered plus a couple of inches. (When sewing on the border, do the usual measuring and mark the border ends instead of cutting the border ends. Leave the extra fabric as an add-on to the border end marks.)
Last edited by Prism99; 10-01-2012 at 11:02 AM.