Is there a special technique for truing flat spoked wheels?
The only real difference is that you want to make sure the spokes don't twist. Ksyriums come with a little fork dealy so you can hold the spoke straight (at least mine did). With others, you'll have to improvise your own tool--pliers with plenty of tape on the jaws so you don't scratch the spokes would do it. Hold the spoke on the flat part nearest the nipple to keep the spoke from twisting.

Otherwise, technique should be the same.

If your wheel won't stay true, you should probably check your spoke tension. If it's too low and/or particularly uneven, you'll never get it to stay true even if you can get a temporary true on the workstand. You want to watch also for spoke wind-up, which is where the spoke twists instead of turning within the nipple and tightening or loosening. Cures for that are proper spoke prep before building the wheel, and a tiny drop of penetrating oil where each nipple contacts the rim. Penetrating oil might help if your spokes weren't prepped properly, but I don't know about that for sure.