The above mentioned plastic polish is good, but it's pretty much about the same as Scratch-X or any liquid polishing compound.
I've always had good luck on all my plastic-lensed cars with just using any lightly abrasive polish - Turtle Wax Polishing Compound, Mother's Cleaner Wax, etc. - rubbing the snot out of it by hand, and then going over it with a long-lasting finish wax (I use F21, but to each their own) to keep it from yellowing out really fast.
They will ALWAYS turn yellow eventually, even after a total resurfacing. One of the reasons being that there's a bajillion little pores in the surface of the plastic from all the road crap that sandblasts it when you drive. Those little pits fill in with dust and grime and such, so in addition to the fact that clear plastic tends to turn yellow, anyhow, those little pits full of dirt only make things worse - adds to the whole cloudiness, makes it that much harder to pass light through 'em.
Polishing liquids and the like will help scrub out the dirt n' stuff, and if you buff hard/thoroughly enough (or if you go the multi-grit sandpaper or buffing wheel route), you can level off the surface so that those little pores aren't as deep and prone to fill with gunk ... although, eventually, you're still gonna have new pits forming on there, and the plastic, itself, is still going to turn yellow.
Happy buffing!