Hide it in the dumbest place you can think of. I put mine in an old grocery bag under some trash on the floor. Thieves look for bags that look like they have a laptop, not a half eaten sandwich and some empty soda cans. A cable lock is good, but making your gear look like literal garbage works better. They want a quick grab, not a dig through last week's lunch.
Well, at least they have their priorities straight. Parker_price might be onto something with the trash bag idea, but honestly, it's just depressing that we have to treat our nice things like literal garbage.
Take it with you, always. I never leave anything in the car, not even in the trunk, because they watch you put it there. If I absolutely have to, I use a plain cable lock through a reinforced laptop bag and loop it around the seat frame, just to make it annoying to grab.