Headlights that are dull and yellow not only look bad, but can be dangerous at night since the lights don't shine as bright as they should. Yellow or dull headlights don't need to be replaced, but they do need to be cleaned. Regular washing won't work, since it's not dirt but oxidation on the headlight lenses that's the problem. Here is a way to clean the headlights until they shine like they are brand new.
Open the hood and place a few layers of masking tape over the edge of any painted areas near the headlights so the paint is protected from the sandpaper that will be used on the headlights.
Wash the headlights with car wash soap or dish liquid soap and a sponge. Rinse clear with water. This is to remove bugs, tar or other build up on the headlight lens.
Use 600-grit sandpaper on the headlight, keeping the headlight and sandpaper wet the entire time. Sand until the headlight looks clearer, even though there will be small scratches on the surface. Repeat this process several times but use a higher grit of sandpaper each time, going from 600 to 800 and up to 1,200 grit. Rinse the headlights and allow them to dry.
Apply the plastic polish to the headlight using the microfiber cloth. Rub the polish onto the headlight over any area that was sanded.
Wipe the headlight with a UV inhibitor sealer to protect the lens from future oxidation caused by UV rays. Without this step, the oxidation will begin to form and build up again in a few months.
Use gloves if your skin is sensitive to polishes and cleaners.