I watched a "sewing with nancy" this past sat. (on PBS) and she had this really nifty little piece of plastic you lay on a corner of your fabric and it helps you make the most perfect (really, perfect!) mitred corner you ever saw! It can be used to make a really thin edge to one as deep as a a satin edge on a wool blanket.
I cannot remember the name :-( , but if you look up her sewing catalog I'm sure she sells it. ) sharet