Step 1: Custom Fit for Your Head

What you'll need:

some duct tape and a plastic grocery bag
a good cutting mat (I highly recommend getting a 24" by 26" one)
a hobby knife (pick up a basic x-acto set if you don't have one)

To start, I laid out lots of strips of duct tape on my cutting mat, and sliced them into various sized smaller strips. Using a plastic grocery bag to protect my head, I started putting tape all over my head to get my personal head-shape. I trimmed off extra plastic as I went. My head was sweating like crazy, as plastic and duct tape don't breath very well...

Step 2: The Lucha Libre Mask Phase

Added strips of tape to cover nose and cheeks, leaving eye holes.

Step 3: Nose Piece

I used a sharpie to draw on the mask (while it was still on) where to cut out to make room for a nose piece. This was made through trial and error with triangular pieces of cardboard folded down the middle until I got one that fit comfortably over my nose. Another triangular piece was taped across the bottom to make it rigid. Then the nose piece was taped into place.

Step 4: Let the Shaping Begin

Once the nose piece was in place, I stuffed the entire thing full of wadded grocery bags and put a few pieces of tape across the chin area to keep the whole thing in a solid, head shape.

At this point I had to just sculpt till I had results I could live with. I used wadded newspaper wrapped in masking tape to get the shapes I needed, and then taped them on. Using a snap-blade style utility knife with a sharp blade fully extended, I trimmed off extra bulk as needed.

I had to put about a six-inch slice in the back of the mask to get it on and off.

The antennae were made of cardboard with newspaper on the inside to add bulk.