Recently, I got into buying costume jewelry and this is what I do:
Go to your local hardware shop and buy a shiny, clear lacquer spray. Don't buy the matte gloss spray. This is a permanent, vinyl or acrylic barrier between the metal and the environment.
First off, get a very good quality costume piece. The key to looking good with it is to make sure it looks the part. Pick something that has a good heaviness to it. If it has gemstones, make sure they are actual prongs, and not glued on.
I tend to like gold-tone pieces, so I get something that is already blemish free, no spots, cracks, or any bit of silver showing through. Make sure it is absolutely spotless before leaving the store. Now, go home and wash that piece, removed every last trace of oil, dirt and debris. Dry it immediately with a thick paper towel, or other lint-free towel.
Prepare a small area for spraying. Spray the jewelry very lightly on the first coat. Turn it over, and do the same on the other side. Wait an hour. Spray another light coat. Turn it over. Do it again. Do spray so much that your jewelry is dripping. Let it dry. Enjoy! You'll never have to worry about it again.