Here's a link from a paint company that has some ideas for faux painting a patina...
I have just used a wash of dark paint if I want to get rid of the "new" look, then spray with a sealer.
I've seen people sand areas with sand paper. Create rust in small areas where you have sanded. Use a thinned coat of dark wood stain over the entire item and it will definitely look as if it had aged that way.
I usually paint the base with a dark color metal primer. Spray some metal finish on top and sand to rough it up. Go over it with a stain or wash to make it look darker in some areas.