Distress the surface lightly with a rasp, file, or hammer. Go slowly as you work. Remember that you can always make more scratches, but you can't remove them.
Treat brighter metals like copper and brass with a chemical antiquing solution, available at most hardware stores. Follow the manufacturer's instructions, and always wear heavy gloves when working with antiquing solution.
Treat metals like nickel, which have a duller luster, with a diluted stain. To make the stain, mix one part dark brown stain with one part water and lightly brush the stain onto the metal with a disposable rag. Take care to work the stain into any dents and scrapes you created when you distressed the surface.