Murphy's Law states that sooner or later you will lock yourself out of your vehicle. Looking inside at the keys sitting on the seat or hanging in the ignition will not get you any closer to being able to drive away, but it will tell you that you need to unlock a car without a key. A simple tool to get inside any locked car is carried around by most tow truck drivers and locksmiths. It would be helpful, and cheaper, if you had access to this tool whenever you needed it because unless you have one handy, you will need to call someone and that means a small price must be paid for the service. If you have a "slim jim" handy, then here's how to get inside that car right now.
Grasp the unlocking strap (slim jim) so the hooked end of the strap is toward the bottom of the tool. Lay it flat against the driver's side window so that the end of the strap is above the bottom edge of the window frame. The window has two rubber strips that run along both the inside and outside of the window. These help the window slide up and down in the middle of the window frame. You will need to slide the unlocking strap in between the window and the outside rubber strip of the window frame in order to get the unlocking tool into position.
Slide the slim jim with the hook side down into the door's mechanical insides using the window as the path to the mechanism. The slim jim should be at least 30 inches long so the tool can reach down far enough to grasp the locking bar that is keeping the car door locked. The tool will encounter resistance to any more downward insertion once it has reached the locking mechanism parts.
Poke the unlocking strap up and down inside the door when the tool has met with parts inside the door near the lock and its mechanisms. As you hit different parts with your poking, you will want to pay attention to the locking and unlocking button inside of the car. For most new cars, the unlocking button is at the top of the inside door near the window or by the door handle inside the car near the front middle of the door. Watching to see if the unlock button moves or is jostled by the slim jim is important because you can see when you hit parts that affect this mechanism.
Slide the unlocking strap from side to side. Thinking of how the hook is interacting with the parts inside the door, once the slim jim has caught the lock and unlock bar to the door, a quick upward pull of the strap should be enough to pull the locking bar up to unlock the car door without a key.
Attempt to find the unlocking bar with the hook side of the slim jim. When a door has a lock that needs a downward motion to unlock, it may be necessary to turn the slim jim over and use the V-shaped end to do some pushing down on the door parts to unlock. Continue to look at the unlocking button inside of the car to determine when you are working with parts that make the button move or jostle. Have a friend keep an eye on the inside of the car to tell you when you are hitting the right parts inside of the door.
After sliding the slim jim inside of the car door into the locking mechanism, try to quickly slide the tool into the parts and pull upward quickly. Because of the way the tool works, it is possible to get it done quickly with some luck.
In some states, owning a personal slim jim is looked down upon by the law enforcement community. Leave it to an automotive locksmith and you won't have anything to worry about.