If you must leave a key with a parking attendant, leave only your vehicle's ignition key.
Install and use anti-theft devices, whether an alarm or a protective device like a club or a collar. Be sure to activate the device every time you leave your vehicle.
Park only in well-lighted areas near other vehicles.
Keep valuables out of sight, preferably locked away in the trunk.
Don't hide a spare key in a magnetic key box; thieves know all the hiding places.
At home, if possible, put your car away in a locked garage, or at least parked in the driveway.
If you have two cars and one of them is easy to break into, then park it in your driveway so it's blocked by the other car.
When you park your car, turn the front wheels to the left or right and put the emergency brake on. This locks the wheels, making it difficult for a thief to tow your car.