In third-world countries, the leading cause is cataracts. This is caused by ultraviolet light (i.e. being outside a lot). Here in North America, replacement of cloudy eye lenses is a routine operation but this is not so in, say, Afghanistan. Still, wearing sunglasses is a good idea; just make sure that they have proper UV filtering.
The 4 leading causes of blindness (usually in old people) are cataracts, macular degeneration, diabetes (diabetic retinopathy; the tiny blood vessels get damaged throughout the body in diabetics, which is why they live an average of 15 years less than non-diabetics (left handed people live 7 years less so if you’re a left-handed diabetic… my condolences)) and glaucoma.
Incidentally, high blood pressure can also damage the small blood vessels in the eye and cause blindness. So, high blood pressure, diabetes and glaucoma (excess fluid pressure in the eye) cause blindness for sort of similar reasons.
As for prevention (and speaking as a person who wears glasses, has a mother-in-law with macular degeneration and two parents who had lens replacement due to cataracts), I suggest:
. 0) as with all health-related things, the most important thing to do is to pick healthy parents and grandparents. Genetics, you know.
. 1) get a regular eye examination every year or so
. 2) wear sunglasses when outside, but make sure they block 99% of UV radiation (and are rated by a reputable agency; there are scams out there).
. 3) stay healthy (keep your blood pressure down and don’t become diabetic through bad eating habits).
Here’s a good reference I found:
http://www.fda.gov/fdac/features/2002/202_eyes.html
(you did, however, happen to ask a question that I actually know something about due to experience).