I use balls of newspaper and tape to get the shape, and then cover it with a layer of paper mache (wall paper paste works, dip the paper in and wad it up to make it more flexible) I use something heavier than newspsper for all but the last layer, use smaller pieces in the detail areas. (Kraft paper is great) after the first layer dries whittle in the detail and add little bits of wadded paper to get the shape you want. Add a few more layers of paper. Dry with a fan between layers (or hair dryer if you are in a hurry). paint with acrylic.

All the masks in my pics (Including the Java Cow) were made this way. Bigger stuff can be roughed out with card board.
The really big skull was made using a big balloon as a starting shape.