About a million ways but not many of them last for long.
Whatever you do, I'd recommend removing the grille from the car to make it easier to clean. Clean thoroughly with some sort of all-purpose cleaner and dedicated brush. Then apply a product. Some of the them are gels which blacken the plastic but wear off in a few weeks. The other option is to look at say crystal resins or "nanotech" stuff like g-techniq, both of which are more difficult to apply but last much, much longer.
