Oops! Updated info: Threads has made the mitered hem lesson part of their Insider section, and now you have to pay for a subscription to view it. Never fear, though, sew4home.com has an excellent tutorial here.
One of my favorite sewing magazines, Threads, has a great website with lots of information on garment construction. Mary Ray's lesson on mitering lightweight fabrics is excellent. I have used this technique several times with great success. This photo is taken from the website. It's a better example than I can photograph at home. Click the link below, and you'll be mitering in no time!