I know there are ways to repair headlights (plastic lenses) with toothpaste etc.,

but what is the best way or best coating to prevent the plastic from hazing, etc.?

I have not looked at FoMoCo's recommendations yet, I am looking at "real world" methods out there.

New lenses are "crystal" in appearance and may need extra care too.
I also have older trucks with fluted type of lenses that need looking after real soon.

 

It would really depend on the environmental factors to your hazing. Fine and beach sand work like sand paper against everything and you will need an ablative coating to protect the lens. I don't know how temperature stable the 3m Protect film is, but that would be an option to stop fine scratches from getting to the lens.

I've always had decent luck with wax. Plain cleaner wax, applied a couple times a year gives a barrier to the actual lens. It probably helps with UV too, but it doesn't last very long. You'd probably need to apply it every month for real protection.

 

They do make kits that are designed to take care of plastic lenses. Try autogeek I use a polish that is designed for sunbeds. They give great protection from UV because they are designed for sunbeds and they have a 4-5 stage polish system but i just you the final one. It works so well that it takes out the hairline scratches from my gloss b pillar and makes them like new.

Here you go Headlight Lens Care: Polish out scratches & haze to restore clear plastic headlight lens covers. Wolfgang, Meguiars, Plexus, lens cleaning system,

 

I have an 01 Ranger, and my headlights look new. Every time I wax the truck, I use Meguiars Cleaner Wax on the headlights.

 

Yellowing is caused by the UV rays from the sun. Maquires products have UV protection in them. Like the last guy said, just apply each time you wax your vehicle and they should stay nice and clear.