Former bicycle mechanic here....
I have built easily over 100 wheels. It is vitally important to make sure that before you install the drive sprocket and rag joint that the wheel is not only properly trued, but also that the spokes are properly and EVENLY tensioned. Park Tool makes a tool called a Tensiometer. It comes with a chart that lists all the appropriate tension ranges for spokes based on spoke size and shape.
As a bicycle wheel turns with a rider on board, the spoke tension changes. The spokes that are parallel to the ground see their tension increase, while the spokes that are perpendicular to the ground see their tension decrease. If some spokes are significantly set at a higher tension while the wheel is unloaded, when you add rider weight and the shock from an uneven surface, these already over tensioned spokes stand a much greater chance of breaking. Once one spoke snaps, it's neighboring spokes need to pick up the load. If any of those are already over tensioned...snap. And the cycle will continue.
If a wheel cannot be properly tensioned and trued, then the rim or hub itself might be damaged. The most common thing I have seen is the rim itself is either out of round or is bent. The test for this is to take an unlaced rim and lay it flat on a flat surface. The rim should make contact all the way around. I tap on the rim in a couple spots to see if it rocks back and forth at all. The other test is to measure the diameter of an unlaced rim at 4 points around the rim. All the measurements should be the same or at least very close.
Check out
www.sheldonbrown.com. There is a wealth of wheel building and related information there.