Where are the spokes breaking? Spoke tension isn’t the only part of this equation, but what it sounds like to me is that the wheels are NOT being built properly – in particular, the spokes aren’t being “prebent” to conform to the spoke line, and/or not being strain relieved during the build (intentionally and briefly overloaded). If both of these steps are done correctly, the spokes should outlive you. I have a set of spokes on a mountain bike that have outlived seven rims (I just swap the rim as they wear out from rim brakes and retension).
Here is the short version of what happens with metal fatigue. Consider a straight length of stainless wire held between your hands. You can bend it a certain amount and it will return to its original shape (elastic deformation). If you bend it more than that, it will yield and take a new shape (plastic deformation). The region where it deforms elastically if further divided into two regions, in the first one the metallurgy does NOT change as you flex it. If you flex it further, the material work-hardens and will eventually fatigue and break.
Here is the counter-intuitive part – if you want to flex it in the shape where it is undergoing fatigue you can instead bend the spoke (plastic deformation) once into that shape permanently, and then afterwards when you flex it the further deformation is elastic and in the region where no hardening occurs. This is usually referred to as “strain relieving” the material.
During the wheel build, the builder needs to perform a number of steps to deform the spokes in this manner and strain relieve them so that during use they do not undergo further fatigue.
See http://sheldonbrown.com/wheelbuild.html and scroll down to “Seating and Stress-Relieving the Spokes”. I prefer to use a lever to bend the spokes to conform to the hub and the spoke line early in the build process, and then after reaching full tension grabbing the spokes in groups of four and squeezing the heck out of them to relieve them. I then retension and do a final truing.
I’ve seen Bronson’s work and he builds a good wheel. I build all my own wheels (and a good number for friends); any mass-produced wheel that I’m faced with I would first loosen the spokes up, check/correct the spoke lines as necessary, and then retension and strain-relieve. Btw, I typically run 1200N tension on all of my unicycle and tandem wheel builds. Quality stainless spokes have no problem with this high a tension, it is more of a function of whether the hub flanges and rims can handle it.
You should never have to tighten spokes once true. Be sure the wheel builder is treating the spokes with something like the Wheelsmith spoke-prep (or boiled linseed oil). When building a large wheel (eg, 29er, 36er) care must also be taken that the
spokes don’t twist (wind-up) as the wheel is tensioned, or that they are untwisted as a final step. I typically mark the spokes with a marker near the nipple so I can see if they twist. The last step is to turn the nipples in the loosen direction to untwist them.