Try attaching some wood, like a 1x2, behind the hole. First using a drywall saw make the hole a regular shape. Just cut the piece of wood slightly larger than the hole and screw through the drywall on either side of the hole into the piece of wood. This will provide the support for your patch. Now measure the hole and cut a piece of drywall that size. Screw the patch in place to the 1x2, tape and apply compound.
MikeyS, July 2008