When I was a kid, I use to break clutch cables all the time. A VW mechanic told me what I was doing wrong. When I installed a new cable and started adjusting the wing nut, I'd just keep turning the nut until I had the correct amount of pedal play. What I didn't realize was that the cable itself was turning with the wing nut. This cause a lot of strain on the cable as it wound it up and it would eventually break. Now I always put a set of needle nose vise grips on the cable just before the threaded end to hold it still and then start making my adjustment. I haven't broken a cable since and that learning lesson was around 20 years ago.
Jason.