Sometimes, it doesn't matter how safely you drive. You could be driving the speed limit and obeying all traffic rules and someone else can crash into you. One good rule of thumb to use is, "Assume everyone else on the road is an idiot." In other words, be prepared for unpredictable lane changes, sudden stops, unsignaled turns, swerving, tailgating and every other bad driving behavior imaginable. Chances are, you'll eventually encounter someone like this -- and it pays to be ready when you do.
It's impossible to list all the possible things another driver might do, but there are a few common examples. If you're pulling out of a driveway into traffic and an oncoming car has its turn signal on, don't assume it's actually turning. You might pull out only to find that turn signal has been blinking since 1987. If you're approaching an intersection where you have the right of way, and another approaching car has the stop sign, don't assume it will actually stop. As you approach, take your foot off the gas and be prepared to brake.
Of course, being prepared requires awareness, so make sure you check your mirrors and keep an eye on side streets so you'll know which other cars are around you and how they're driving. Don't focus only on the road in front of your car -- look ahead so you can see what's happening 50 to 100 yards (46 to 91 meters) up the road.