If they're broken, buy new ones. If they're just a tiny bit cloudy or all the way to yellow, rough, crusty and looking like they're beyond repair, just sand 'em and buff 'em.

You can start as coarse as 320 grit dry (but the lenses had better be totally opaque before you have to go that far) then move to 500 and 800 dry, then 1000 wet and finish off with 3000 wet. Then polish them with anything from PlastX to almost any polishing compound and you're done. Which grit paper you need to start with depends on how far gone they are, but it's amazing just how far gone these things can be and still come back to life.

Once you've brought 'em back, either recoat them or just use some plastic polish on them once a month. It will take you a whopping 60 seconds per light every month to do this, so if you're too busy to give one minute a month to each headlight, well.......