Polishing headlights is ridiculously easy, however they always seem to yellow again after a couple months (or less). The reason being is that by polishing the headlights to remove the yellowed plastic, you remove what is left of the ultra violet (UV) OEM coating on the plastic. So, to fix it, you need to add some form of UV protection back to the headlight lense. Here is how I did it.
I started out with Meguiar's PlastX, and a cloth buffing wheel on my battery drill. That gets rid of the yellow oxidized layer of plastic on the headlights. Its pretty quick work with the battery drill.
Here we have one light done, and the other obviously not done. I had just done these about 3 or 4 months ago and you can already see how bad they were without re-adding some form of UV protection.
To add that UV protection, I grabbed some spar urethane. You add some mineral spirits to cut it. Use a 50/50 mix. Then dab a cloth in the urethane and rub it on the headlight. You barely need any to do the lights as this stuff is thin. I used a quarter sized drop of each on a yogurt lid and that was more than enough.
And of course, here are some of the before and after pics of the light to show how pretty it is now.
I haven't seen how long this lasts. I've read it'll last 2 years. I actually would like to put another coat or two on since its so thin. Seems like that would give you a bit more time.