I'm a very absentminded ham. I have a radio mounted in my car, wired directly to the vehicle battery. Fuses and high-gauge wire and all that good stuff. But, if I accidentally leave it on, my battery will be dead by the next morning! I don't know anything about ignition sense wires or anything like that. What's an easy way to prevent draining down my battery?
If you search for "automatic power off car ham" you'll find a number of solutions which don't require additional wiring. When the engine is running the voltage supply is higher than when the engine is off, so these work by sensing when the voltage is high enough, and turning the radio on.
Some radios, such as the Yaesu FT-857D, have a feature in the menu that allows you to set the unit to automatically power off after a user-chosen period of time. Search the documentation of your radio for such an option.
Research automotive relays; They are essentially electronic switches that turn on when a current is present. In my vehicle, I have a large gauge wire and fuse running from the battery to high power pin on a relay under my seat; A small gauge wire runs from the back side of the cigerette lighter (which is only powered when the car is on) to the relay coil feed (trigger) pin, and another large gauge wire running to a fuse box that all my radio equipment connects to. It effectively creates the ignition sense you seek, and you will learn about the science as you go along!
I have cited this resource in the past; It's thorough, and it applies well to this task: http://www.mp3car.com/the-faq-emporium/117895-faq-relays-how-they-work-and-how-to-wire-it-up.html
Double-check to see if your radio can auto-turn off after a period of non-use - mine does and it's quite handy. Failing that, a product like http://www.westmountainradio.com/product_info.php?products_id=apo3_pp can work quite well to do something similar based on the voltage from your alternator/battery "detecting" if your engine is running or not.
You might increase the capacity of the battery by replacing it with a higher rating, or add a second battery. This will allow you more time before it is drained. It also increases the time you can use the radio in an emergency where you are unable to run the engine.