Back in the 80s and early 90s when my '69 was my daily driver, I had a crude, simple means of locking the hood shut that required a key. This helped to keep my shaker from going places I didn't want it to.
For the whole car, my other means of protection I used were: the Club (may not be highly effective, but it
is a deterrent), always locking doors, and an alarm system with motion sensors/engine kill. When leaving the car someplace I was really uncomfortable with or leaving it for any period of time, I also would open the hood and disable the coil wires, then lock the hood back up--my theory was that maybe the thief would get frustrated with a car that wouldn't start and bail on the attempted theft.... Of course if they see you messing under the hood it is a giveaway

. The alarm system was always a pain in the rear as it would go nuts at 2 or 3am for no reason, but I had it nonetheless.
My current theft deterrent was to completely disassemble the car (See avatar). I call it "The Discombobulated Immobilizer" system, the thief just won't know where to start with stealing this heap of parts--patent pending of course. It really works great!!! I will be licensing this out to others--PM if interested in a license agreement or franchise.
As someone else stated, if the thief wants your car bad enough there is no stopping them. A rollback is the easiest way to steal a vehicle--just watch Overhaulin and observe how people's neighbors watch cars disappear without saying a word.