NJC,
There are a few things that have helped DS6 with this. The first, is to practice practising. What I mean by this is ask your DD to do something just for practice. When she's done, crumple it up and throw it away. When she complains tell her that she was just practicing practice. It sounds crazy but it works. Once the kid realizes that there isn't any point in doing their best work to throw away, they take the pressure off of themselves.
Even as I read it, it still makes no sense but it worked for DS. The second thing I recommend is a book called "ISH" written by Peter Reynolds? I think. It is a very short book that defines perfect as different to everyone and it's written in such a way that your DD will get the point loud and clear. We read it every night for weeks.... DS squirreled it away and read it in bed at night.
Good Luck!
