Dedicated always-on 12 volt accessory plugs wired to the battery (with 10A fuses) - front and rear. <$15 and an hour to do both.
I need to do this - I recently added a 400w inverter ($15 off Craiglist, + maybe $30 in wire & misc parts) under my the shifter console. I basically copied NLXTACY's install. I lucked out and got an inverter that also has a USB port, so charging things like cell phones & mp3 players is a breeze. This also frees up my one lonely 12v outlet, but like you said - it's not a constant source of power, and it's only one outlet.