Turning the water heater back on after a long vacation, I happened to adjust the temperature control so that the hot water for the shower was consistently just right. No cold water needed. Just turn on the hot.
Through this chance discovery, the hot water heater now burns less gas, and there's no longer a need to fiddle with the cold and hot water knobs before and during the shower.
Not to say this will work in all homes. Serendipity may be playing a role here, involving the rhythm of hot water use in the house, but for the sake of some simplicity and economy, it's worth a try.
Update, Feb/2012: This approach continues to work very well, for everyone in the family. The logic is that the hotter the water in the tank, the more heat is lost through the tank's walls, which means more energy needs to be applied to keep the water at that high temperature. This past summer, we left the water heater on low after returning from a vacation, and actually found the coolish showers refreshing during hot weather.
Adendum: Electric water heaters may be a little harder to adjust, as the temperature adjustments are typically behind metal covers, and there may be separate adjustments for upper and lower heat coils. Lots of tutorials can be found via internet search, with this link as an example. Since electric water heaters are less efficient than gas ones at heating water, learning to adjust them is worth the extra effort.