Before you worry about protecting the lenses, you need to restore them so they are nice and clear... all headlights come from the factory with UV protection on them, but this will wear away overtime and the plastic will oxidize as you can see on the vehicle you are talking about. You must remove this oxidation to bring clarity back to the lenses, and you must then apply a new layer of UV protection or else the lenses will oxidize again very quickly.
The first step is to wet sand the lenses down to remove the oxidation and create a new, fresh surface. (if they are not that bad, you may not need to sand). In the photo, I used 2000 grit paper. You can see that the entire lens has been sanded and is now a uniform surface.
Next, a cutting compound like Meguiar's M105 and cutting pad must be used to remove the sanding scratches from the surface and begin to restore clarity to the lens. This 50/50 shot shows the difference in the compounded area and the sanded area.
Once the entire lens has been compounded, you can refine the surface even further with a finishing polish like Meguiar's M205 on a polishing pad.
Now that the lens looks like new, you should protect it with a durable, long lasting protection like Opti-Lens headlight coating. This method of protection will outlast any traditional wax or spray protectant as the Optimum Coating will truly bond to the lens and create a very durable layer of protection. It should last years.