Give all of your attention to what is happening on the road. A split second is all that is needed for things to turn ugly, and if you're not aware of what is going on, you could get into some serious trouble.
Don't let your passengers distract you, even ignore them outright when you need to. If someone complains that you haven't been listening, just ask them if they would rather get into an accident. Most people will accept this, especially in busy traffic -- if they don't, ignore them anyway -- it's your hide you're protecting, too.
Turn off the cell phone and don't use it while your are driving. Some people like to talk and talk and talk while driving. These people shouldn't be driving. If you must leave your phone on, pull over when taking a call - seriously.
Many accidents happen because someone made a surprise turn. Make sure that everybody knows exactly where you are going, even if you don't see anybody. Just because you don't see someone, that doesn't mean they won't end up in your path. You might be surprised how many people will cut you off because they assumed you were going in another direction. An accident is automatically your fault if you didn't signal.
Make sure you turn the signal off when you are through. Don't depend on the steering wheel to turn it off for you. Turn it off yourself. You might be surprised how many people will pull out in front of you because your signal was left on.
Hitting that little lever is a habit into which you can easily train yourself.
A car with its headlights on is seen instantly. Turning your headlights on gives you a strong advantage. This doesn't mean you should always have your headlights on. You should turn them on when visibility gets tough. If you need to turn on your wipers, turn on your headlights as well. If the sun isn't visible, turn them on. Dawn and dusk are particularly dangerous.
If the sun is at your back, turn on your lights. Look at the glare in your back window. This is what oncoming traffic is trying to see through. Your headlights help them see you through the glare. Don't worry about adding to the glare of the sun, as your headlights can't possibly add anything more to that brilliance.
Always leave 2 seconds of space between you and the car in front of you. What does this mean? When the rear of the car before you passes a point in the road, the front of your car shouldn't pass that point before two seconds pass. Why? If you're any closer, then sooner or later you're guaranteed to take part in one of those "multi-car-pileups" that you hear about on the news. If the car in front of you stops suddenly or hits something, you will need those two seconds to be able to stop before smashing into it.
How do you do this? Look for a landmark such as a telephone pole, a signpost, or a crack or line in the road. When the rear of the car before you reaches this landmark, start counting, "1001, 1002". Don't count too quickly -- that's a losing game. The front of your car shouldn't reach the landmark until you finish saying, "two."
Some other drivers seem to think that this amount of space between you and the car in front of you is plenty enough for them to squeeze into, especially on the highway. Just pull back when someone does this. Don't let it bother you, as you really won't be any more delayed by this. It's better to keep your distance than - surprise! - become part of his trunk.
Get some of those yellow sunglasses. Not brown or orange or red or even pale yellow, but pure yellow. These are sometimes called hunter's glasses or aviator's glasses. They cut out the blue light that is most responsible for poor visibility during bad weather and glare from the sun or headlights. You'll be surprised how well you can see through fog, through rain, or at night with these.
Learn some basic physics. Your car is a huge weight that will go in a straight line unless some force acts upon it. Treating it as such can give you much more control and confidence over ice or in an emergency.
You know that other people have "body language" that you can read to tell what they are feeling or what they are about to do. Your car has a body language, too. You can give other drivers an idea of your intentions by how you drive and how you position your car. It's a subtle effect, but it can give you a defensive advantage. Just don't overdo it and be a bully, especially if you drive a truck. This is also not an excuse to not use your signals. Learn it and use it.