Once its dry, put a bunch of cotton batting (available at the fabric store, or inside of an old stuffed animal) inside of the top of the mask. Add a little bit by the chin. The padding should hold the mask onto your head where you want it.
The rough texture of the unfinished papier mache inside the mask should hold the cotton batting in. You can also try gluing it down. Alternatively you could build some sort of frame inside similar to a hard hat.