Drill a small hole in the bottom of the lens to relieve condensation; fresh air will alleviate and prevent condensation.
Apply white toothpaste to a soft clean cloth and rub the lens with it in small circular motions. Rinse the lens with clean water before the toothpaste dries.
Spray carburetor cleaner on a dry clean cloth and wipe the lens thoroughly.
Spray WD-40 on the lens and polish the lens.
Apply rubbing compound to the lens with a soft clean cloth and buff the shine back with a separate dry clean cloth.