I'm a big fan of homemade Halloween costumes and look forward to seeing all of the creative costumes the kids in my neighborhood come up with. Each year my son proposes a costume idea and he and I do our best to execute it. One product, which I picked up at a hardware store, has come to the rescue for the past three years running and has so much potential to be a part of scads of different costumes:
Aluminum foil tape. Bam.
Typically used for sealing air ducts, you can use it in costumes to mimic metal (think robots, armor, vehicles, etc.) or to add shine and bling. It comes on a large roll, like shipping tape and usually has a peel-off backing. It goes on smooth, but will crinkle a bit over time.
I bought mine at a neighborhood hardware store for around $6/roll. If you have trouble finding it locally, you can buy it online. Silver contact paper will give you a similar result and is a good choice for larger costumes.