Deb Karasik has a great pictoral tutorial:
http://www.debkarasik.com/store.php?cat=5
When she demonstrated it to us in class, she has added another step - at page 11, where she is showing the marked line for the miter, she now stitches that line down, which further secures the mitered corner (I have not tried that yet so I don't have a comparison as my current method is very similar to her tutorial).
I always do a little blind stitching to sew the corners so that they don't gap.
Cheers, K