I think the trick to building wheels that don't come apart after a while is to release the torsion on the spokes. Obviously, when you tighten spokes they twist as well, but if you leave them like that when you use the wheel and it flexes the spokes straighten and the nipples loosen as a result. I've built three wheels, and the first one came undone because I didn't let off the torsion. Going back half a turn for each whole turn does the trick, or flexing the wheel also works, I think.