A quick fix is to simply polish them with rubbing compound and polishing compound on a terry cloth rag. My father did this to his 03 taurus and it really worked well, but it doesn't last long.
What needs to happen is the oxidized (or crazed) plastic removed entirely then the headlight sealed with a UV resistant clearcoat. I've used this method myself several times on six cargo/third brake light lenses and my mother's headlights. Here, this video is what I use specifically and it turns out very well. Just don't expect to be done within 5 minutes. This takes time. Give it a day for each headlight from start to finish.
3M also makes a headlight restoration kit, which can be seen used by EricTheCarGuy also on Youtube, particurally on a Subaru, a Chrysler Town & Country minivan, and a Ford Contour, the latter of the two within the Fixing It Forward series. Pic your poison, although I prefer Chris's method.