You could also try making something for the wig to hold onto yourself. I something like a band made of hot glue that goes around her head. I learned this from
http://www.youtube.com/watch?v=asyyOQ17b9sThe bad thing about it being glue is that after a while it will stretch to exactly fit the head that makes it not hold. The good thing though is that i just blow dry the seam just enough to make it malleable and fold it in together to make it tighter again.
Hope this helps