If you had the chance to install a $600 piece of equipment that would enable you to use 20% less heated water without compromising shower warmth, would you do so?  Unfortunately, I didn't because I had already closed-in my showers by the time I learned about grey-water heat exchangers but according another homeowner who has installed one the preliminary data suggests significant savings from the device.

The principle is simple: as hot water from the shower goes down the drain, cold water going into the shower goes through a heat exchanger wrapped around the drainpipe and warms up so that less hot water is required to blend with it to reach your target temperature.  You effectively re-use the energy that had gone into the heating the water

If you are interested in this, check out the GFX technology web site at http://www.gfxtechnology.com/GFX.html and read more about a real-world application at http://www.greenerhouse.info/showerheatrecovery.

I also found another company, Heat Exchangers Nf. Inc. who sell a heat exchanger that can be used as a retrofit called the Shower Smart Faucet.  I'd be interested to hear feedback from anyone who's tried it.