My only rule: the bike is locked up unless I'm riding it. This includes in my garage as well as in the car.
In the garage, I lock my bike to my daughter's bike. It's one of those Barbie bikes, but it weighs more than mine does!
In the minivan, I loop a cable through both wheels & frame and then through a loop that is also used for securing the rear seat. Immediately after a ride I put the bike in the car and also lock it up.
At home I'm using an old U-lock with the circular key. This isn't the most secure lock, but the goal is to keep the bike from walking away. The lock in the car is a Kryptonite New York.
None of these precautions will stop a determined thief. The goal is to dissuade a casual smash-N-grab.