Small, almost microscopic burrs on saddles are a major cause of string breakage at that point. Abrasive cord is available in several thicknesses and can be used to smooth the groove in the saddle over which the string passes, and also the notches in nuts.
In reading the contributions in this thread, I'm puzzled as to why some players need to change string sets as often as every performance. That sounds ridiculous, as well as costly. I'm a lead player who bends frequently and doesn't play like a *****. It has been years since I broke any string on any of the guitars I play. I religiously use a device called The String Cleaner after every set. It is a hinged device, inserted opened, under the strings at the pickups, and snapped closed, sandwiching the strings, then slid along the strings from the bridge to the nut and back down again. The strings are sandwiched between 2 microfiber pads which line the device's surfaces and effectively clean all surfaces of the strings. I know how long-used strings sound as opposed to new strings. I have used the same set of strings for several months without breakage or loss of much resonance, practicing this cleaning method.
As noted above, the player's physiology enters into the equation. I rarely have to change my strings and they just don't corrode. But others will turn a set of strings black or green in no time. Corrosion is the primary premature killer of strings, not usage. Sharp edges will indeed do it, but they won't make strings go dead and they always break at the same location.