Use hair-dyer + lots of patience then you can remove any mists in your headlight, provided your headlight seal isn't leaking, else doesn't matter how many times you do it, the mist will just re-appear.
Personal experience: when through flash-flood with crystal headlights, somehow manage to get about, say, a little saucer you use to put your cili padi + kicap, that much of water inside the headlight, result is mistly headlight, so hair-dry plus remove bulb plus stuff cloth inside to soak up all the water, after that continue blow drying till you are satisfied. if mist still appears, continue blow drying. problem solved.
FIY: Blow drying is done on the face (glass) of your headlight, since you removed your bulbs it provides exit way for the vapour to get the hell out of your headlight.
Unless you are talking about in inside of your headlight being dirty then you need to open it up to clean.
By the way how to maintain a clean windscreen? Having 'water-marks' on it, tried with soap, newspaper, windscreen cleaner. Any other options?