What if you got hoops of plastic of differing circumferances (that's probably spelled wrong, oh well), and attached them to the inside of the dress with the biggest at the bottom, smallest at the top - so the dress stands out like a tree.
Then for your head, get some bright yellow felt and make a star to go around your face, or even just get a halo from an angel costume - and be the angel on top of the tree?
And just get lots of those little mini ornaments and hot glue them all over the dress.
If you do this, you have to put a picture up for us all to see!!!!