One of the most inefficient things we do with energy is heat up water with it only to let most of that water (and most of the energy) go right down the drain when we take showers (baths are a little better because more of the energy goes into heating the house in the winter, but they do use more water).
So I think that if we were serious about wanting to save energy one thing we could do is try to encourage shorter showers by making people aware of how much energy they use. What I imagine is a little display that shows you how much energy you've used since the start of your shower (if you like you could factor in the cost of the water and sewage as well as the energy used to heat the water). For maximum effectiveness, I think the value displayed by the meter should be in units of local currency, and correspond to the amount you'd spend taking a shower of that length every day for a year - that way it feels like you're spending money really fast (of course, you'd really only be spending it at 1/365th of that rate so it's a bit of a psychological hack/cheat).
I think many people would appreciate such a device for the money it would save them, though I suppose some people might believe that long, guilt-free showers are worth that extra money.