Here's a simple way to make cheap, fake metal props that will not easily bend and crease. I had heard styrene sheets were a good material for costume armor and weapons, so I thought I'd give it a try.
This is a method commonly used by cosplay costumers. Most of my research brought me to Amethyst Angel's tutorials, which go into greater detail for more specific and complex prop builds. Here I simply cover some of the basics of this technique.

Step 1: Getting Started

Tools:
Pencil
Scissors
Paintbrush
Hot glue gun

Materials:
Styrene Sheets - Most people recommend .20" thickness for flexibility and ease to work with. Smaller panels can be found at
     hobby stores, but for larger sheets you will have to shop online.
Acrylic paint - Gold, Silver and Black (or any other colors you want to include). Black is used for weathering.
Paper - to plan out your cuts

     I started by planning out the crown's design on paper, to make sure everything was symmetrical and fit together nicely. Since I was using smaller styrene panels which I found at a hobby shop, my design was made in several smaller pieces that would be assembled in a repeating pattern.
     When I was happy with the shape of things I traced the design onto the styrene sheets and cut them out. When everything was assembled, it looked pretty decent. At this point the crown could be bent into a ring with the flexibility of the plastic. It still looked somewhat odd and flat, so I decided to try a little shaping.

Step 2: Shaping and Assembling

     I laid the crown pieces on a curved surface, a large cardboard tube, and used a heat gun to soften the plastic to press down on into a curve. It proved to be more difficult a step than I had expected. The styrene did not gradually become malleable as I had thought it would, but rather it suddenly went limp and curled up when I brought the heat gun too close. Eventually I was able to save my mistakes and get something decent looking.
     I assembled the crown and fixed everything in place with hot glue. At this point I was able to see how the finished product would act, and made adjustments accordingly. The thin segments of plastic going around the back were too flimsy not to flex and sway when the crown was moved (it was meant to be more used as a prop than a costume piece), so I added a band of corrugated cardboard around the inside for support. Had I the time I probably would have invested in a prettier building material, or added some coat hanger wire to keep things stiff, but the cardboard would have to do on short notice.
     I wanted to add some kind of raised pattern and fake rivets to the crown, so I decided to draw them on with hot glue (again a trick I learned from AmethystAngel) before painting. If you have patterns that are close together, work in passes and allow the glue to dry before adding on, otherwise your lines could run together.

Step 3: Painting

     I started painting with a layer of black as a base coat. This was mostly because if I missed a spot later I didn't want it to show as bright white. Also, the metallic paint I used needed several layers before getting a solid color. I found that a shoddy layer of gold on top of a solid black lent a nice effect at a glance. I did this for the inside of the crown in place of a solid color and weathering layer, just to save myself the time and effort.
     When painting the crown, I found that the hot glue designs tended not to hold paint as well as the plastic. A careless brushstroke would not spread paint onto the glue, and could sometimes wipe of previous paint layers as well. Only gentle dabs of paint worked on these areas.
     When the metallic colors were dry I glued the (now painted silver) cardboard band in place inside the crown and started on weathering. To do a weathered effect, use a half-and-half mixture of black acrylic paint in water. Depending on the metal you want to replicate, you may want to use black and green, to simulate oxidation. Brush the watered-down paint onto a small area and allow it to sit for a few moments, then wipe it back up with a paper towel. Ideally, you should leave behind a bit of paint in the small crevices of the item's shape.

Step 4: There You Have It

I built this crown as a prop for a student film. After gathering materials I was able to complete it in a couple days at a cost of around $7 (I already owned the paint and glue gun).
This was my first attempt at using styrene sheets for building, so I'm still learning to work with the stuff. If you have any remarks or suggestions that could make this instructable more helpful feel free to leave a comment.