I have effectively tuned my guitar down a whole tone to accomodate a variety of alternate tunings and since then I have hardly broken a string. So, why not try a lower gauge string which would not be at such a high tension? Alternatively, identify where the string tends to break - nut, bridge, tuning post because some remdial action to one (or more) of these by a luthier might fix the problem. Do you play in a standard tuning or do you change it about? If the latter, then the continual tightening and loosening of the string will weaken it (it doesn't affect me too badly though because of the lower tension).