First off, make sure that you're working on solid brass with no lacquered coating. Otherwise, this won't work.
The secret to the patina is ammonia. But remember ammonia has pretty strong fumes so I suggest you work outside or in a well-ventilated area, and make sure that you wear gloves.
Fill a container with enough ammonia to completely submerge the piece you want to age. Slip the piece into the ammonia and seal the lid. Then just let it sit. The amount of time you leave it in the ammonia will determine on how much aging you see. Soaking it overnight will give you kind of a varied brass and brown finish. a few days will give you a great rich brown color and five days or more will give you that great green patina! Once you're happy with the results, remove the piece and rinse it off.
Easy ways to age brass - I'm Mrs. Fixit and it's just that simple!