If they are really yellow, it's best to just replace them.
If they are just getting a little foggy then the best solution long term is to wetsand them with 1500 grit, then 2000 grit sandpaper. Then spray them with an automotive clear coat to restore a protective coating on the plastic.
You can also use 3m cutting compound and a machine polisher... DA (dual action/aka orbital) will work, however rotary is best. Then wax them to polish them up.
The reason they yellow is because the factory protective coating on the plastic eventually wears away and the UV rays yellow the plastic.
If you don't want to clear them, you'll just need to wax them about once a month with 2 coats of wax.