I just came across something that looks really interesting.
It has sensors that attach to your skis and work like an optical mouse on a computer, imaging the snow and determining the direction of movement of your skis. It then produces a sound in the earphones when it detects your skis slipping sideways, as opposed to gliding cleanly on their edges, prompting you to correct your technique.
Has anyone tried this device? I'm really curious, but the non-trivial $750 price tag is holding me back (at least until next winter :-)