Borrowing a friend's car may seem like an easy and obvious thing to do, but as many can testify, it can go horribly wrong. Taking a few minutes beforehand to check things through can save you a lot of bother in the long run.
Ask your nearest and dearest only. Family and close friends, especially those you have known for a long time, will probably go out their way to help you. Avoid asking favors from those you don't know too well. The response will either be a resounding "no" or they may agree but feel used and uncomfortable. Either way, this will put a strain on the friendship.
Agree on the terms. If you are only borrowing the car for an hour, this may not be necessary, but if it's any longer, then be upfront. Agree on the ground rules, such as time, mileage and whether or not your friend expects compensation. Be honest about where you will travel to and what you are using the vehicle for. If anything happens, it will be better if you are upfront from the beginning.
Check it's all legal. One of the most difficult things to know when borrowing another vehicle is if you are covered and how. There are no general guidelines, as every insurance company and every policy is different and has different restrictions. Check your own insurance policy to see if you are covered to drive other vehicles and on what basis. Also, check the policy that covers the actual vehicle so you are certain of your legal standing in any event.
Understand the car. For most people who have been driving for decades this may seem like a silly idea, but it's not. If your friend's car is a stick shift, make sure you know how to operate it. If the vehicle is larger than your normal ride, ensure you are able to maneuver it. The same applies to newer luxury vehicles that may have a multitude of sensors and warning panels that you should be able to recognize.
Take care. Be more cautious than you are when driving your own vehicle. Stick to speed limits at all times and pay special attention when changing lanes. Don't drive distracted or drive when tired.