I did paper-mache armor for my Alis (Phantasy Star) costume -- itturned out very nice and was fairly durable, the problems I ran into was that the glue I used on the wires that held it on me popped off (hot glue did NOT work.) Also glaze both sides or it does weird things underneath, and line it with felt, I had an "armor hickey" from the collar rubbing on my neck.
I basically took a plastic saucer sled, did about 10-12 layers of paper mache over the whole thing and then made a template of cardboard of the exact shape I wanted, cut it out with ginzu shears (no joke, my friend has the ones from TV) glazed the whole thing with 2-3 coats of Elmer's Glue (you can also use gesso, which is mucho expensive and requires sanding) but the Elmer's took the paint nicely. A coat of clear spray paint to prime, 2 coats of base black, I hand-painted the gold accents, and 2 coats of clear to set it all. I did a combination of gluing it to the base dress and wiring the bottom part so I could get in and out of the costume. Allow several weeks for the entire process, the paper mache itself is a couple of weeks to do properly.
This is the result (thanks Kevin!) Not in this pictue the collar is crooked, I fixed it later as it was kind of like a necklace I laid over the chest plate.
If you have any questions, plase e-mail me and I will try and explain in more detail how I did this.