I'm doing a class/demo this Thursday about using those cheap lighter plug USB charger adapters. For a dollar, you can get a complete switch mode regulator circuit that is about the size of an LM7805 with heat sink, without the heat or inefficiencies.
Did you know that, from a very low current to the maximum rated current, a linear regulator's efficiency is Vout/Vin? So 12V into an Arduino with the on-board 5V regulator has an efficiency of 5/12 or about 41%?
But a switch mode regulator has an efficiency of 60 to 95%, usually lowest at very low current draw and above 85% over most of the range.
Come to the meetup this Thursday May 21 at 7pm. http://www.meetup.com/OlyMEGA/events/222437259/