It depends how elaborate you want to go. Simple Kill Switch can run you about $5-$10 dollars. Basically it goes between the ignition and the starter, when switched off no power will go from the ignition to the starter. Place it in a area that is not common to hide one (like fender well) but some place you can reach too. Others have a battery cutoff switch which shuts power completely to your car, but you loose all settings for radio. I have both Battery Cutoff and Kill switch. Diagram below.
{BATTERY}-- (+)--------ignition system lead---------*SWITCH*---------starter