I have found that a gram of prevention is worth a kilogram of cure, use something like AutoGlym super resin polish on headlights every month or so ( and other lights as well ), you can follow with a wax for extra UV protection if you want. This adds to the original UV protection and puts an extra layer of protection on to prevent build up of traffic pollution and dead flies on the plastic, which under the merciless rays of the sun will degrade the surface. This works best if started when car is fairly young, if the lights are already clouding up repeated applications of super resin polish may help but not guaranteed.
Autoglym super resin polish is great for cleaning up UPVC window frames - use it once a year to keep them looking great and keep the spider poo from sticking. I originally bought some Autoglym to clean window frames and then decided to try it on headlights ( I am not a great car cleaner ).