I saw this post and the follow up on fakebook.
Another person said that the cause is usually corrosion on the headlight switch circuit board. She showed a picture of hers that started smoking. She was lucky and able to prevent it from going up in flames.
Prevention might be done by making sure your drains are clean! Any overflow of water could be a potential hazard. Also if you leave your car out and it's going to rain, buy a good car cover.
I think Valk, has a great idea as well. There should be a way to put a fuse between that switch and the power source.