Every time we take a shower, we are literally flushing kWH(/btu/joules) down the drain.

(from Hot Water)

The amount of energy you use in the shower depends on the following:

time = shower time in minutes

rate = shower rate in gallons per minute

temp = shower temperature in degrees F

baseTemp = your tap's "cold" temp, in degrees F

Take those values, and use them thusly:

time x rate x (temp - baseTemp) x  0.00244 = energy in kWh

That gives you the energy embodied in the water you're flushing down the drain.  You can get some of that energy back! (perhaps after you've turned down the temperature on your water heater and invested in a high-quality, low-flow show head :).

One trick is to use a drain water heat recovery system (DWHR, or sometimes GWHR).  These are sections of copper piping which replace some of your outgoing drain pipe, and extracts some of the heat to preheat the water coming into your house.

More info here:

• General info: http://oikos.com/esb/49/gfx.html, EnergySavers.gov
• GFX, one such product, made in the USA: http://gfxtechnology.com/ 
• PowerPipe, another GWHR product: http://www.renewability.com/power_pipe/index.html

Note that DWHR is unsuitable for applications which are not "equal flow".  If the hot water isn't flowing down the drain at the same time that the warm water is being drawn in & heated, then DWHR can't operate*.  This precludes it from being useful for things like baths, dishwashers & clotheswashers.

* there are systems where a preheat tank is used to cycle water through the DWHR system when the drain is flowing but the heater is not, which can still recover some heat, but these seem relatively complex, and I can't find efficiency numbers for them.

There's at least one of these products (need to find it) which simply uses drain heat to warm the incoming cold water just before it enters the shower controls. The problem there is the assumption that the user is mixing cold water into his shower.  If your hot water is hotter than you need for a shower, it's probably too hot.

Based on the ideas above, Soren opens and closes the drain during his shower.  The first "round" (before getting in) leaves the water quite cold from the tub (but the tub much warner).  The second round keeps feet warm and radiates heat upwards to warm up the air.  Warmth from below makes having the cut-off valve partially closed much more pleasant.  Except at the end during the final rinse, most of the warmth is captured from the water before it goes down the drain.