Water conservation seems to be on everyone's mind lately. It seems to me that a fairly simple way to encourage people to use less water during shower would be to offer a convenient way to "pause" it. See, I'm all for turning the water off while soaping up, but it's a pain to get it going again at just the right temperature and pressure that I like -- and I think that the majority of people are the same way. When I'm soapy or have shampoo in my hair, I don't really want to fiddle with the taps to get the right temperature again, and I think that's the reason why most people just don't bother.
If there was a convenient switch that would let me shut off the flow of water but keep it at the right temperature and pressure the instant I turn it back on, I'd use it every time.
(This is not the same as shutting the water off automatically after 20 seconds. I've had an opportunity to use those a couple of times in Europe, and they are just annoying, as they shut off when you least want them to, like when you have soap in your eyes).
Seriously, something like a big knob on the wall. Pull it and the shower pauses. Slam on it, and it's back on again. I think people would actually use that.