I have a prox alarm in my 03 vert with a lojack system. Guess so far that it's
working as no one has attempted to steal the car.

I believe MustangWorld.com has a good How-To on putting a switch
between the emergency fuel cutoff in the trunk and hiding it. When
switched, no way your car will be going anywhere under it's own power.
The wires run under the driver's side door sill so it's pretty easy to
hide a switch under eg the seat.

I just purchased one of those micorwave sensors that are supposed to
make a security bubble around the car. Haven't installed it yet tho.
Don't have the time for it at this point as I intend to do it at the
same time I install power door lock actuators...

These microwave sensors can be had on eBay for about $15...

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Handling: Upper strut brace, 17" wheels, K&N Airfilter
Styling : Mesh grill, Bullitt fuel door, aluminum gauge insert

Hi:

I went to an "inner city" high school, where kids did steal cars(albeit
25 years ago), and I've worked in mentoring some of these types.  The
only thing that I know they cannot figure out is the fuel pump cutoff
that I use in my cars - hidden in the open.  These kids go to
"disneyland" (juvenile justice center), and come out knowing how to
disable just about any alarm out there, cut your wheel if you have a
club, and they'll steer with a vice grip if you take your steering
wheel. One kid I did some things with several years ago told me about
his brother (he's in trouble now - 15 years old) modifying some gizmo
devices (gizmo is the brand name of the device)- they allow you to
bypass the alarm system to remote start - these kids are smart and the
money is GOOD - an escalade is worth about 3-4K hot at a chop shop to
the kids, ever notice how many of these are stolen, despite transponder
keys and onstar?  Juvenile justice is a school for some of these kids,
and they are bright, although criminal, and the money and training is
out there for car theft.  In order to stop this stuff, we have to get
the chop shop people, the kids who do the car theft do it for money, and
can defeat the lojack, onstar, transponder keys, etc etc etc., and
standard alarms don't phase them.  One kid told me about 5 years ago
that the only thing that would stop him from getting my car was if I had
a snake in it - he's behind bars now (good thing), but none of the kids
that I know can get my car more than a few feet - fuel pump cutoff and  
hide the switch in the open.  I'm both glad and sad that I don't deal
with these kids anymore.

Andrew  

Add a double pole switch and use it to disable the starter circuit and the
fuel pump circuit.
No one will be able to start the car or push start it (won't protect from
being towed).
Find a place to hide the switch where only you know where it is.  Hiding it
in plain sight is the best solution.  The places and ways to locate a
disable switch are limited only by your imagination (mine is located where I
can discreetly activate it with my fingertips without anyone realizing it).

LJH
95GT

Interior movement sensor with a pager.  Lets you summon help without tipping
your hand to the thieves.