This is almost too simple, but it might be a good hint for somebody

I was cleaning and greasing my headset today and noticed that water and road peepee could get into the headset quite easily.

I took two slices of tire inner-tube cut the round-way like a doughnut and put them around the upper and lower bearings where moisture could get in.

If you want something more fancy, Lizard skins makes headset covers.