Check your mirrors and gauges constantly
This is driving 101, and frankly, if you're not already checking your mirrors every few seconds, I really don't want to share the road with you. Assuming you've adjusted your mirrors properly, scanning them not only becomes habitual, but you'll be unconsciously aware of the relative locations of the vehicles around you. Tracking your gauges means you're more likely to anticipate if something bad is about to happen mechanically -- and at the very least, you'll never have to worry about how far it is to the next gas station. This is called situational awareness, and it's kinda useful when all hell breaks loose.
... and your blind spot before you put your blinker on
Er, you do check your blind spot and use turn signals, right? If you're perpetually scanning your mirrors, you should already know no one's there, and checking is simply backup. You should never rely on your car's sensors to tell you if the coast is clear, and if your car ever has to warn you of an unsafe lane change, you've failed at this.