I built a small DC-DC converter that provides a regulated 12v output from the cigarette lighter (which can be as high as 14.5 volts or so, and sometimes drops below 7v when starting the engine).
http://www.eegeek.net/mp3box/DCDC/index.htm
it's a buck-boost converter, uses a pretty standard, cheap controller chip, and is a pretty straightforward design. The design is not my own; When I built it, I was starting from scratch with DC-DC converters, so I followed this guy's design:
**broken link removed**
But, I certainly learned a lot along the way.