It helps if one lives in California right now, but anyone can see the need for water conservation is increasing these days. If one lives in an apartment, cutting back on shower water is about the biggest individual thing they can do to help.
The simplest way to reduce water usage in the shower is to eliminate it; either using a sponge bath, or a system like the fog shower proposed by Buckminster Fuller - essentially a home-use pressure washer for humans. While either is capable of cleaning someone with less than a gallon of water (just a fraction of an ordinary shower), both leave one standing naked and freezing in their bathroom.
I propose a modest-sized steam unit for the residential apartment. Something strong enough to turn a typical shower enclosure into a comfortable steam area. Bonus points would integrate a high-pressure fog hose. But the key thing is to keep people warm, creating a steam that exfoliates, relaxes, and keeps all the feeling of a shower without the waste water.