Cheap way to clean plastic lenses?

Ha! got you all to think not this question again:can:

Its a lens question but not lasers:D It's about the plastic headlight lenses on cars and the yellowing issue's.
Yes i'm a car guy but all of my rides have always had the glass lenses so I never had the need to try cleaning them. My friends car has this issue though and instead of paying $20 to $30 for a polishing kit or what not, i'm wondering if anyone has tried any home made remedy's like toothpaste or baking soda?