I ALWAYS double lock my car doors. If I use the key to lock the doors, I twist it twice. If I use the power door lock, I push the button twice. If I do it by hand, I always run my hand over the lock again to ensure it's fully pushed over.

I also make sure my front door is locked at all times, even when I'm home. I don't like knowing someone could get in :P

That's all I can think of at the moment..