Buckle up. Both you and your passengers should always wear a seatbelt. In most states, wearing a seatbelt is mandatory. If you do have an accident and someone in your car is not buckled up, you could be charged with a crime.
Allow plenty of room between you and the car in front of you. In other words, back off.
Slow down. Never underestimate the power (and weight) of a vehicle speeding down the roadway. This may be why many fatalities from car accidents have been linked to speeding.
Watch the intersections. Over half of the crashes in urban areas occurred at intersections. Drivers who run red lights are a major cause of serious injuries.
Drive defensively. Assume that the other driver is going to make an illegal maneuver, and be ready to avoid the problem.
Forbid drugs or alcohol in your car. It's illegal in all 50 states. If you know your friends are using or carrying drugs or alcohol, do not allow them to ride with you. It is as simple as that.