I am reading Mark Beaumont's "The man who cycled the world". He recounts how he had 2 (was it 3) spokes break at the beginning of his journey. In Poland, he had the wheel completely rebuilt. The mechanic replaced all the spokes with thinner ones and loosened the tension on them. He explained that the spokes were too over-tensioned to bear the weight of his load. I haven't finished the book but he's made it to Pakistan (which is as far as I have read) and did not have any more broken spokes.