Both of these are wonderful tutorials on mitered borders, that board members have put together for us :D:D:D
If I was going to mitre multiple borders, I would sew all 4 borders first (sew all of the side pieces and top and bottom together) and then treat them each, as a single border. That way you are only mitering one border instead of several.
If you first sew the mitre with a very long basting stitch, you can quickly check to see if you are right on... if not? leave your thread tails long and just tug on the top thread and they come undone lickety split :wink:
http://www.quiltingboard.com/t-47911-1.htm
http://www.quiltingboard.com/t-21057-1.htm