Another way is an easy one but might cost your some money but if you are a friend to one of them it might be free. If you happened to know someone working in a car painting workshop, ask them to polish your headlamps glass/plastic with their Paste Compound polish, this type of polish is what they use to polish your car that has just been painted with new paint, they should know.
However, i encounter some older glass headlights which is not the crystal type, which has lines on the inside of the glass, usually gets yellowish/greenish, the only way to clean this inside of the glass is to open the headlamp and wash.