I've only polished my headlights twice over the last year, and they still look good as new. I polished them with Mothers Plastic Polish, and I seal/wax them monthly. I think as long as they are kept protected by a wax/sealant, they will not turn yellow.
I think GSRStilez did a great step by step of restoring headlights a while ago. You might do a search for it. He does great work and presents the info very well and uses pictures to add to his presentation, so that what he is doing is very clear. A very talented young man.