Your best bet is an alarm system. Or you could just unplug all the spark plug leads every time you get out of the car, they'll be scratching their heads for ages....
Not very...
As far as making it unstealable, I'm not sure. But I can suggest an extra alarm that alerts you from your keyring when anything happens at your car, with gps so you can find it if you don't get there before they steal it, and a way to find out who took it if they are lucky enough not to be there when you find it again.
As far as details...I figure the less public the details are the more likely they are to work
Satellite tracking will be your best hope to recover the car still together, but in saying that who is going to look twice at a tow truck driver towing a car and fitting a car cover over it to protect it. cover the car with lead blankets will block out the GPS signal.
Point is you will not stop a pro who wants your car only deter a amateur, use a clublock fit a alarm and after market immobiliser
there was a towie in perth a few years back that was going around knocking off new statesmans and storing them inside his shed. the got him for the gps alert on one of the cars. suppose you could even rig up a genuine switch you dont use as a kill switch. they would never find it if you had it like in a rear demister switch
like above post, if your desperate, you can disconnect your plug leads. I moved to a pretty dodgy area of the city, and some nights, if i dont get a good parking spot, i remove my leads completely, and bring them inside with me !!!!!