I am hopelessly absent-minded. Coupled with the fact that I usually get to school early and often sit in my parking space listening to the radio and mucking about on my computer for a while before going in to class, I often leave the key in the ignition and/or the door unlocked.

Clearly, rather than just trying to remember, I had to find an unnecessarily complex and technology-oriented solution.

Even if you aren't as much of a scatterbrain as I am, car theft is a problem. Alarms are universally annoying and universally ignored. On Star and Low Jack are expensive. How do you avoid coming out to your parking space to find a bit of broken glass and nothing else? Make starting it a complex and arcane ritual beyond the comprehension of the uninitiated.

Step 1: Figuring Out the Circuit

I puzzled over this problem for some time. One option I've heard is wiring a toggle switch into the fuel injection computer. This would likely confound any thieves attempting to steal the car, as it wouldn't immediately suggest the use of a traditional killswitch. The danger, of course, is that if the switch were accidentally turned off in the course of driving, the engine would die. In my car, a 2002 Toyota Celica, I realized that there was a switch under the dash which prevented the ignition from engaging when the clutch wasn't depressed fully. By interrupting this circuit, I could prevent anyone who didn't know about that switch from starting the car.

Step 2: Choosing the Switch.

If someone breaks into your car and tries to hotwire it, they're probably familiar with theft prevention measures like this one. Therefore, it is important to conceal your switch. There are a few ways you can do this; if there's a hiding place that's easily reached from the driver's seat, that's one option. However, this too is probably easily discovered by any but an amateur thief--there are only so many spots one can hide the switch and still keep it accessible.
As such, I personally decided to hide the switch in plain sight. My Celica has a button in the dash which prevents the passenger window from rolling down. This is an entirely useless function--but it's a toggle-style switch which would not look at all out of place. Other rarely used switches could also be used--though I'm not sure of the legal implications, one could use the emergency flasher button. If you don't have any switches you don't use, consider wiring another switch to control the function of something you do use and assigning the original switch the killswitch function.

Since the whole point of installing this switch, for me, is that I often forget to remove the key from the car, betting that I will remember to turn off the toggle switch each time I leave the car seems imprudent. I therefore decided to install a momentary switch as well--even if I leave the toggle switch activated, the momentary switch still prevents theft. If both switches are open, it's a double line of defense.

Step 3: Rewiring the Old Switch

This step is specific to my project, and may not be an issue for you. I found that when the window lock button was depressed, it broke the circuit which controlled the window. Therefore, simply removing and re-purposing the switch would prevent me from ever rolling down my passenger window again. To fix this, I bridged the contacts with a small twist of copper wire and taped it in place. that allowed me to push it back into the dash and forget about it.

Step 4: Wiring the Switch

I used speaker wire to hook the two switches into the killswitch circuit. It's cheap and easy to work with. I wired them in series, as it were, so that either switch being open would prevent the car from starting. Though the first and fourth contacts were connected for the window lock circuit, a few moments with my ohm-meter revealed that the third and fourth completed a circuit when the button was depressed. I connected the wires to these contacts, cutting away a part of the switch's housing to facilitate this.

Step 5: Installation

Having wired the two switches together, it was time to put them into the car. I took one of the blank panels set into the dash (which already had a switch controlling the inverter which provides 110v AC to the power outlet in what used to be my car's ashtray) and drilled a hole for the momentary switch.
I put the window lock switch back in its place and fed the momentary switch through the back of the panel. Then, I cut one of the leads into the existing clutch pedal kill switch and stripped the ends. Then I connected them to either end of the security killswitch circuit I made with wire nuts. It worked like a charm.
Any questions?