Could be a multitude of factors. Weight versus number of spokes. Some wheels simply aren't suitable for certain weights. Spoke tensions could also be all over the place, causing them to break. If you have them re-built then ask around and find a decent wheel builder. A proper wheel builder will put a small brass washer on each spoke when he re-builds it (at the shoulder), will test the spoke tensions with a tension meter and ensure the right length spoke is used. I'm 13 stones and use 32 or 36 spokes on the back wheel and, touch wood, have never had a problem.
I don't profess to be an expert. I've done a wheel building course but that doesn't qualify me to have the answer, I'm afraid, but if it's built properly and is appropriate for your weight then you really shouldn't have to worry about spokes breaking. It doesn't have to cost a fortune (I'm a tight wad so I should know) but at least you won't have to buy new wheels. One thing you could suggest to the wheel builder is try a heavier guage spoke. And they're getting a pack of 36 for under a tenner, by the way.