First, I have never tried any of these methods. The headlights I've seen in need of restoration were so bad off (yellowed and pitted), replacement was the best option.

For those NOT as far gone, the trial these guys at MightyCarMods tried looks like a pretty fair test of many methods people have heard (and some I know I never heard of).

 

Linseed oil also works!

Thanks for the post Jason!