Dr Green's post above gives some excellent advice. I would like to point out however that most low flow shower heads are "aerating" which means they introduce air thereby causing the smaller water droplets to cool off much quicker. So yes, in a sense you are using less water but you could be using more hot water.
It's best to find a low flow shower head that is non aerating for maximum energy efficiency and water conservation, such as these...
http://www.nuprana.com/HE_Handheld_S..._p/n2945ch.htm
FWIW;
Dan