If you start from brand new, 303 protectant is about the best stuff going to keep the plastic from fading but eventually ol' Sol has his way and the UV rays do their thing. Once they are beyond protecting with 303 the best approach is to clean them real well with a solution of warm water with Dawn dishwashing liquid and when they dry, wipe them down with a body solvent such as 3M general purpose cleaner and spray them with a satin black trim paint. Krylon, Duplicolor or SEM are the best choices for this.
Mike