In my case, I'm convinced that the reason the spokes kept breaking was that the spokes where way too loose. They broke at the elbow, and that probably means that every time I put power to the wheel, half the spokes tensioned and the other half went slack. When the power goes away at the dead spots in the pedal stroke, the ones that were slack slam back to tension. It beats on the elbow part of the spoke.

When I built my own wheels, I took a friend's advice and cranked the tension as high as I could without stripping the outside of the nipple with the spoke wrench, including leaning on the wheel to get them a little tighter. Neither of the wheels that I built have ever broken a spoke or gone out of true in the slightest, despite me riding 8 miles a day over potholed and washboarded gravel for thousands of miles.