A lot of people have been asking wheel questions so I thought I'd give my thoughts on this. (Not that I'm expert)

I'd like to present my theory hopefully to stimulate more questions and hopefully learn something myself in the process.

The wheel is basically a tensioned structure. Spokes on either side of the flange keep the wheel center and true in the lateral plane.

Trueness is lost by one of more spokes losing tension.

1. Any trauma the wheel sees on the road is transmitted through the rim to the spokes.
2. If the force transmitted to the spokes is great enough to cause the threaded end to loosen out of the nipple tension will be lost.
3. Stress on the spokes is cumulative. That is repeated small traumas can lead to progressive loss of tension in one of more spokes (thread loosening)

Therefore as I see it the best way to keep a wheel in true is having the following conditions present:

1. Reasonable tire pressures to reduce shock transmitted to the rim
2. Stiff rim to reduce flex and stress delivered to spoke threads (where the spokes loosen out of the nipple)
3. Use butted spokes instead of straight guage spokes. Having the center section thinner than the threaded section limits the force a spoke can deliver on it's threads. Threads loosening is why rims go out of true.
4. Spoke prep: Some like it. Some don't. In any event it helps keeps spoke threads from backing out in the
event of a bad combination like a flexible rim/stiff spoke set-up.

I'm sure someone here will have an opposing opinion and I'd like to hear it.

EDIT: Loss of true can also happen if the spoke pulls through the rim (caused by fractures around the spoke holes. Rim is junk at that point)