I personally use Meguiar's Natural Shine Protectant. Spray it onto any black plastic or rubber trim, and work in with a microfiber cloth, rag, etc. You might have to do this several times to restore the piece to "like new" condition, but the end result is a piece that looks like it just came out of the factory, without painting it (I don't believe in painting plastics to restore them if there is a better way to achieve the same look while keeping everything as-is and factory).
If that still does not work, try Turtle Wax trim restorer, Mother's Back to Black, or any equivalent product. I personally use the Turtle Wax variant (not a fan of other Turtle Wax products, though) to restore trim that is extremely weathered, but it is messy stuff and leaves behind residue after washing, so I use it as infrequently as possible. As long as you wipe up any excess a few hours after applying it, you should have little to no residue next time you wash the truck.
You can find all of these products at your local auto parts store or Wal-Mart, and they should all be under $10. I can post some pictures of my bumper moulding tonight if you would like to see the end result of my method. Good luck restoring that trim!