I have used prickly pear pads for TREATING sunburn (more for relief than anything), not sure how good of a preventative it would be.
The Apache also used it for all kinds of wounds, according to Eve Ball's book In the Days of Victorio.
I have read that comfrey leaf, jojoba oil, and African shea butter are good natural sunscreens, as well as octyl methoxycinnamate from cinnamon or cassia oil. I know shea butter comes from the pits of green berries on the Kotschy trees in central Africa. There is jojoba everywhere in my stomping ground, but how to extract the oil is a mystery to me.
Beta carotene supposedly helps protect from UV rays, so eat your carrots.