You need to be very serious about protection if you want your circuitry to last, and that includes protection against such occasional insults as battery polarity reversal (e.g. improperly connected jump starts), 125 V "load dump" transients caused by the alternator when battery connections become corroded and fail, and much higher voltage spikes from the ignition system.
Here is a summary from Tyco: http://www.digikey.com/Web%20Export/Supplier%20Content/Tyco_8004/PDF/Tyco_AutoNetwork.pdf
There are a number of transient surge suppressors designed for such applications. I've used this circuit http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Automotive_Filter.png (the 1.5KE18 Zener diode can withstand short term surges of 1.5 kW)
