There are lots of anti-theft devices on the market.
The absolute best is a hidden kill switch. This cuts off the flow of electricity or fuel to the engine until you hit the switch. And when I say hidden, I mean hidden. Your best option is to mount it so that you can move it periodically. Put it under the carpet, behind the kick panel, under the ashtray. What you don't want is a computer-key-operated kill switch with a blinking light that is installed under the dash in plain view. If you know where to find the switch and what you are doing, a quarter will buy you the car. That quarter inserted into the switch will bypass it. The car starts right up.
Alarms, when properly installed, can discourage thieves. The biggest problem is finding a good installer. You should check with the Better Business Bureau and other consumer protection groups to find out about the background of the store you are considering. Ask for references and credentials. In some states, there are no regulations as far as alarm stores go. While it's unlikely, it's possible you could have a convicted car thief working on your car. Also, if it's not properly installed, the only thing it will discourage is good relations with your neighbors when a critter bumps into it at 2 a.m. and the alarm goes off.
Club-like devices, or metal bars that prevent the steering wheel from turning, will keep the 10-year-olds away. An experienced car thief will cut the steering wheel or pour something on the bar that will cause it to freeze, get brittle and crack. Or he'll know that some people get lazy and put the bar on without really locking it. The major problem is that most people won't put it on but just once. Then the car gets stolen with the bar sitting on the floor of the back seat.
Another device is a big hinged collar that goes over the ignition lock cylinder. It is more than $100 and has to be taken off and put on each time a person drives the car. The problem with this is that on some cars it plain old doesn't work. It covers everything on a GM column except for the lower bowl, which means it can be stolen. If the lower bowl is broken, the column is then defeated!
Another device out there is called the boot. It is applied to the tire and wheel so the car can't be towed or driven. It's the same kind of deal that parking lots and police departments use on errant parkers. It is available to the public.
There are tracking devices, hidden transmitters that send out signals to police if a car is reported stolen. Some are better than others. Some require that you report the theft to the police, then they activate the search. Then the search might take 24 hours. If there's a time lag between the time it is stolen and the time you report it, what's going to be done to your car is done. Other companies track your call all the time by satellite. That is more effective, but consider the cost. It can be $50 or $600 up front, then a monthly fee of $40 or more.
Then there is the Muffin-like device. Muffin belongs to a friend of mine and has worked without fail for more than a decade. She's a big dog who looks like a wolf. She sits in the back seat and acts like it's her car.