I would say the easiest way is to get the inverter that plugs into the lighter socket.
The better way IMHO would be to make or buy a DC-DC converter, you can look at the DC-DC thread on this board for that.
To make one is a matter of finding the parts, I think Sproggy had plans, you will have to search on this board.
When I get a power supply it will be DC-DC with about 200-250 Watts.
------------------
http://www.geocities.com/mr_bubba_zanetti/