Making a car difficult to steal

A hidden kill switch will take care of the less professional thieves. You can also put a kill on the fuel pump if you run electric.

But in some cases these guys aren't even starting the cars, they're rolling them into trailers or hooking them up with repo-man style tow trucks. I heard of one in So-Cal where the car (a pretty nice hot rod) was blocked by another car in the driveway. The thieves would have needed to use wheel dollies to get it out, and did so while the owner was sleeping in the room next to where the car was parked. If you're looking to stop the professional types, good luck.

Best option there is to GPS it so you can track it once its gone. There are several companies now offering GPS tracking devices for classic cars, although most have an ongoing monthly fee. A lot of GPS cell phones can be tracked by the owner of the phone using software on a laptop or home computer (all iPhones and iPads can be tracked by the owner with built in software), but then you're paying for one of those. And if it drops out of coverage (like it would, for example, in a shipping container), you're out of luck.