Guide to Safe Driving
A basic toolkit might help you fix a problem that would otherwise leave your car stranded. For instance, what if the problem is as simple as a loose battery cable? Having a screwdriver or a wrench will get you out of that situation in no time. The older your car is, the more likely a tool kit will come in handy. If your worn radiator hose is starting to leak or an exhaust pipe is coming loose, duct tape is a great fix-all. Also put jumper cables in your tool kit. Even if your car doesn't break down, they're good to have to help somebody else. Another option for recharging a dead battery is a mini-battery that plugs into your cigarette lighter.
Don't forget to have some rags for wiping up fluids or cleaning grime off your hands. Also, include in that kit a flashlight with extra batteries. If you have to pull over at night, a flashlight is essential, whether you're spraying tire sealant, adding fluids, signaling for help, or having to walk a few miles to the nearest town.
When your car becomes disabled, try to pull over to the right side of the road as far as possible. If it's dark, park under a light or lay down some flares or warning triangles to make sure other motorists (and possibly a tow truck) can see you. More than 3,000 traffic deaths per year are due to accidents that take place during roadside repairs. A triangle is usually better than a flare because it can't burn out but if you can, use both. Your car's emergency flashers are good for a limited amount of time, but they will eventually run down your battery.
Links to...
Safe Driving Teen Drivers
Saving Gas
Defensive Driving

Things essential...
In the "Must have" department, a cell phone is great for contacting a tow service, but that's not the only thing you need to carry in your car, especially if you're stuck in an area where there's no cell coverage. A can (or two) of tire sealant is one of those easy-to-use, quick-fix items that can get you rolling again in minutes, instead of struggling with a jack or waiting for a tow truck. It not only can plug a hole, but also inflate the tire. Keep in mind, though, that these products are only effective on minor punctures. You still need to carry a spare (don't forget to check its inflation level regularly) or have run-flat tires.
Just in Case
Under the "Not a bad idea" heading, the items you put in your vehicle may depend on your climate and geography. For hot, desert areas, bottled water (a gallon is best) is a must. In cold weather, a blanket is not only good for warmth, but also as a ground cloth should you need to crawl underneath your car. A pair of gloves (the soft kind that allow for manual dexterity) can come in handy as well.
Some drivers who take long trips on a regular basis like to carry along spare accessory belts and an empty fuel jug (don't store fuel in an enclosed area where flammable fumes might accumulate). A fire extinguisher is great for handling minor fires, but use good judgment if the flames really start to engulf the vehicle. You're better off to file an insurance claim on your car than a medical claim for burn injuries.