I have been doing this for as long as I can remember, but I just recently saw this topic come up in the automotive section of the local newspaper. Someone wrote in and asked how they could keep the headlights on their vehicle from hazing and yellowing. The answer was simple... wax them! I laughed to myself and thought, "Wow! I'm a genius!" since as I said earlier, I've been waxing all the exterior lights on my vehicles probably since I was 16 and started washing and waxing our family vehicles.
I guess this would also explain why the headlights, taillights and fog lights on both our 2009 Vibe GTs still look brand new!
So, to sum up... wax your headlights, taillights, fog lights, etc. and they should stay nice and clear for a long time to come !