How do you keep cats from jumping up on your parked car?

Cats hate the smell of citrus. If you can get a citrus-scented car wax, you may be able to keep them off for weeks at a time.

EDIT: Citrus is fantastic for keeping cats away from something. For several years we piled oranges around the base of the XMas tree. The only problem is that our cats all get disgusted when we eat an orange over the holidays and thus are no longer available to pet cats, and our laps are no longer pleasant to sleep on.

Lay strips of masking tape sticky side up on the portion of the car the hop up on. Cats dislike having anything stuck to their feet and is easier to remove than stronger tapes like duct tape or strapping tape. After having been "taped" a couple of times, the behavior should stop. I tried it once to keep a cat off my guitar amps. It worked for me!

Spray it with Febreeze, 409, Lysol, or other cleaner. Do this every day when you park it for the night. You can also spray home made pepper spray on it, but be sure to dilute it. Boil a can of jalepeno peppers in water for 20 minutes or so, add a quart or more of water. Using gloves, carefully pour into spray bottle.

chain a bunch of dogs to the top of your car. Have them bark ferociously at all times and that will deter the felines.

I don't think there could be away as cats always like to jump onto the bonnet when the engine is still warm.
YOU have to be careful though as some cats go under the car and climb into the engine its self- I now this because my sister had this happen and it wasn't a pretty site when the car was started up.(It was awful)

park in a garage

set up an elaborate sprinkler system surrounding your car.

or douse it in cat nip.