You can use dressings such as Back to Black, etc.. Or you can repaint them. That'd be the route I'd ultimately go, considering dressings do little more than make them look fresh for a little while and fade back again.
I've also seen guys that scotch pads to the trim then use a dressing. It seems to allow the trim to take the product and hold it better.
I've tried the Back to Black, and it really works. Though mine started to fade again after a few months. I've heard that 303 products are great protectant. Anyone have tried this?
I repaired the plastic bed rails and wiper cowl before we traded our 05. A can of Krylon Fuzion in satin black and 45 minutes made a world of difference.