If you're going to go the route of a SMPS, I'd recommend using a chip designed for SMPS control. National (national.com) has a number of these ICs. For the voltage range you are talking about, I'd go with a flyback topology. They are relatively straightforward, but do require some familiarity with electronics and the design equations. If you aren't familiar with them, I'd go hunt around on the web for flyback power supply designs, and start understanding how they work (lots of website out there they talk about the threory of operation). I'd also search on here for other threads about flyback converters. Also, a book on SMPS design could be very useful, if you are trying to build something from scratch.
I found this article in a quick search. It would be worth a look.
**broken link removed**
It's meant to run from a 12V battery, so you'll have to verify that the design can operate from 10-20V at the input without burning up any of the devices.