Like culzean I usually try to give plastic headlights a regular gentle coat of Autoglym Resin. It is very (very) mildly abrasive and will clean slight yellowing but also probably wears down the UV protection on new units so use discretion. Unfortunately it is a downhill slope, once they get to the point of beginning to yellow, the UV protection is more or less gone and you need to do it more frequently. My old Yaris was 20yrs old and needed doing every month or two especially during summer. It saved the cost of replacing the headlights but became a bit of a chore.
