use a heat gun or if you dont have one, use a hairdryer.
just heat the plastic up by wafting the gun over the bumper or black trim and it kind of burns the top layer of polymer ions causing them to go black again. This will last as long as the black lasted out of the factory, dont worry about doing it too much either, it doesnt actually burn away layers of plastic.
if you use a hairdryer rather than a heat gun, then obviously the process will take longer