Knowing how to shut off your water supply in an emergency can prevent extensive damage and save you money on repairs. The first thing to know is the location of your main shut-off valve, which is usually housed close to your water meter. If you can familiarize yourself with the exact location of this valve ahead of time, you'll be able to more quickly find it in the case of an actual emergency.
The next thing to consider is whether the valve you found is accessible and functional. Sometimes, a main shut-off valve may be very old and may not work as well as it did when it was first installed. If you have an old valve, routine inspections will help ensure it's easy to turn when you really need it. If the valve is accessible but somewhat stiff or corroded, you might have a valve that's just on the verge of seizing up. Also, consider labeling the main shut-off valve so that it can be easily pointed out to someone who's not familiar with your home.
One other vital piece of information is the plumbing layout of your place. Even in such an uncomplicated system as ours, there are multiple shut-off points.
Being familiar with the valves and their corresponding shut-off commands can make a huge difference in a moment of crisis. I can't imagine anyone would attempt to live without knowing the shut-off locations and commands for the system they're dependent on.
And what's more: This is life safety information. If you don't know where the commands are, the whole system is rendered useless in a time of need.
Finally, keeping an emergency toolkit with vital tools like a wrench can make a huge difference when you need to shut off water lines that are stuck or screwed on tightly. Know where your toolbox is, and make sure it's readily accessible. Run practice drills that are as realistic as possible. Everyone in your household should have a clear and preset role to play in getting the system shut down. You might be tempted to call this a useful fire drill, but it's really more like a useful tsunami drill. Staying informed about local plumbing codes or state regulations can give you a lot of insight into the kind of additional measures you can and should take.