If you are talking about the rubber ones (which I have) I spray on a product called "Bleche White" and scub them with a wet brush. Hose off the grime. I spray the flaps with the same tire foam I use on my tires after they are dry.
If you have the plastic ones, I use a product called "Black Chrome", but I don't know if it's available anymore. It works well on all the black plastic (mirrors, bumper tops, window moldings, etc. ) I use it after the truck is dry.
Hope that helps.
Juan