I use Solvol Autosol on mine.. it's a metal polish.. then follow up with a paint restorer (like T-cut) and finally toothpaste or something really mild.
The yellowing comes back really quickly once the protective coating has faded away, so to keep it clear for longer you need to apply some acrylic clearcoat (like the top coat "lacquer" when you paint the car).
If your car is a left hand drive, and you want to keep it for a while, look at buying brand new headlights. "Left hand drive" ones are still available.. although I don't know how much longer for.
Ralf S.