@zaku: I was making a bad joke… I wouldn’t want my cat to be shot by an angry car owner. However to answer ‘heyu…‘s question, I would firstly suggest a cover for your car. It’s not that much of a hassle to put on and can also keep other stuff from getting it dirty.
If a cover isn’t an option, maybe a motion-sensor activated sprinkler nearby, a super-soaker water gun, or to go extremes, I have heard that horseradish concentrate at a very diluted level can keep cats away, if you wanted to douse your car in that every night. Personally I think a cover would be much easier =P