1.) Have a properly inflated spare tire or donut in your vehicle available for use along with a jack and tire iron. I would also keep some work gloves and a few towels in the trunk to aid you when you need to change a tire.
2.) Have some road flares or reflective triangles available to place safely behind your vehicle should you break down to keep other drivers in "the know" that you are disabled and on the side of the road. This could help prevent your vehicle from being struck from behind.
3.) Have a cell phone charger in your car that runs off your cigarette lighter in your vehicle.
4.) Keep a quart of oil and some anti-freeze in your trunk, you never know when you may need them.
5.) Throw and extra jacket or sweater in your vehicles trunk
6.) Keep a few bottles of water in your vehicle to be used in the event of an emergency.
7.) Invest in one of those battery charger/inflators. These things are fantastic and you need to make sure to keep it charged or make sure it can plugged into the cigarette lighter (won't help with a dead battery). You will be ok if you take it out every few months and ensure it is charged.
8.) Keep a small first aid kit in the car incase someone hurts themselves (like when they are changing the tire or lighting the flares).
9.) Always may sure you have some cash or at least a credit card with you in case you need to be towed. Most towing companies can be real pains when it comes to this and they won't tow without something up front.
10.) Always have a decent set of jumper cables available, they have saved me on a few ocassions and I have also been able to help a few folks out as well.
...that is all.
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