mild dish-washing soap, water and an old tee shirt.
if you can/want to take it apart: soak it in the bath tub in a soapy solution. just avoid abrasive cleaners and pads and high pressure water.
true patina won't come off... just dirt.
something i've been doing lately with pretty great success is using goop, or fast orange... you know the hand cleaners that you use when you get all good and greasy.
DO NOT SCRUB WITH THESE!!!!! as they have ground up pumas in them. just take the bike apart, and soak the parts in a tub or sink with some water and hand cleaner mixed up in it. this will do a good job of breaking up oily patches that trap dirt. my guess is that it is the orange oil or folic acid in the hand cleaners that is doing the work.