We have a good number of useful threads about this. Most recently, see my reply #16 here: http://board.marlincrawler.com/index.php?topic=95822.msg1068808#msg1068808

Quick search returned these useful threads:
Easy anti theft device: http://board.marlincrawler.com/index.php?topic=19808
Anti Theft kill switch ideas: http://board.marlincrawler.com/index.php?topic=33062.0
WHAT DO WE DO: http://board.marlincrawler.com/index.php?topic=61977
My truck got stolen....: http://board.marlincrawler.com/index.php?topic=6465
Why do thieves like Toyota trucks in SoCal?: http://board.marlincrawler.com/index.php?topic=18951

Search the forum for more.

BigMike


EDIT: I've added the quote from my "reply #16" linked above:

Locking the pedals together is a pretty good idea.

Wainiha, information on kill switches has more or less been kept to a minimum as to limit info from would-be-thieves. It's not rocket science so I'll summarize with few details that there are two main methods: One is to interrupt the circuit of any device that is required for the engine to maintain running, such as disabling your igniter, or coil, or fuel pump, or EFI circuit, or ECU, or air flow meter, or injector resistor(s), et cetera. The second method would be to interrupt the circuit of any device that is required to start your engine, such as leveraging your clutch start cancel feature, or leveraging your Park mode feature (automatics, ie. transmission must be in park or sometimes neutral in order to start), or disabling your starter, or disabling your actual ignition switch, et cetera.

The operating principles of the two methods are as follows: If you disable whatever it is that keeps your engine running, then one could still crank and turn the engine over, it just won't start. I call this a "Noisy Kill Switch". If you disable whatever it is that actually starts your engine, then that's it, the engine simply won't start. I call this a "Silent Kill Switch".

Benefits: The benefit of a Noisy Kill Switch is that you will hear your engine cranking repeatedly at 3am which will alert you to the fact that some douchebag is trying to rip you off. The benefit of a Silent Kill Switch is that you disable the kill device to start the engine, and then once it's started you re-enable the kill switch. That way whenever you park your truck, you aren't reaching all over the place flipping switch(es) all the while wondering who is looking at you ... because you would have already enabled the device before you even left your driveway.

Draw backs: Draw back to the Noisy Kill Switch is that every time you park you'll need to enable the device. Someone might see you enabling it and then they would know where to look. Draw back to the Silent Kill Switch is that because you enable it after the engine is running, if you accidentally stall your engine at a green light, you'll have to scramble to flip your switch to disable it, fire up the engine, then flip it back to re-enable (well, this step is optional but its good to keep the habit of doing so), then proceed to drive forward while a bunch of traffic behind you are honking and upset thinking this is your first day ever driving a stick shift.

Some tips: I have both types on my vehicles and I only use the Silent one on a day-to-day basis because I like getting out of the vehicle without having to enable anything (I've already enabled it). If I am parking my vehicle for an extended time, like parked for a week while I'm away on vacation, then I'll also enable the Noisy switch. And here is the tip: Have a kill device located outside your cab. The criminal is going to get into your truck and initially sit in your driver's seat to try to start it up, or at least be in the driver's area. Well, if your kill switch is for example mounted above your spare tire, he can do everything he tries, even pop the hood and try to hot wire everything under the sun and he will never get it to start. I'm not a thief but I assume every minute counts and adds a lot to their pressure. If they can't figure it out within 5 mins I bet they will just move on. I could be entirely wrong about this though, but at least it prevents the passer-by type thief who "thinks" he found an easy target in a Safeway parking lot with grandmas loading cat food into cars next to the one he is trying to rip off. I bet they give up real quick if they can't figure it out in a short amount of time.

For carburetors, you can install a manual check valve inline with your fuel line, and with it switched to the off position, the engine will fire up and run, but it will only run long enough until the fuel in the float of the carburetor is consumed at which point it will just run out of fuel and stall (maybe they could get a quarter mile away).

Another cool idea is what many have done including KYOTA and BLACKDOG and that is to weld two chain half-links onto your transmission and transfer case shift handles (or Crawl Box handle if you have duals) in such a manner that you'd shift the transmission rearward into reverse and the transfer case forward into lowrange, and then connect the two links together with a hefty Padlock. Pictures:
BLACKDOG, from reply #17:



KYOTA, from reply #426 (I thought KYOTA had a better image of this but this is the only one I could find, you can see his chain links welded to his first two shifters):


Now if the thief ever managed to get your truck running, he'd be limited to Low Range in reverse -- and even more important is that the truck could not be shifted into neutral to push it away since it'd be physically locked in low range + lowest transmission gear = very hard to move the truck. This wouldn't apply to dual/triple case setups as there would be another lever that could be shifted into neutral, but still a nice feature to have. Maybe some thieves push the victim's car down the block before they start messing with it. Well this would prevent that from happening.

Hope this motives someone to do something to safeguard their truck
BigMike