Keep in mind, once you buff out the headlights you have removed all of the UV protection that there was.....and as the factory UV protection dies it turns that brown/yellow. So....once your buff it out, there is no UV protection left and the light will get worse MUCH more quickly than it did before, and start cracking. The only way to prevent this is re-apply some kind of UV protection (think spraying a UV Clear Coat paint type product on them). Some places, like headlight doctor do this as part of their restoration service.

Good luck.