1. Be prepared - keep a spare cell phone battery, blankets and food (carbohydrates) in the car
2. Stay inside the car, (unless you absolutely know where to go and that you can make it) but make sure periodically you are able to get out and renew airflow.
3. Run the motor for 5-10 minutes each hour
4. Make sure the exhaust pipe is not blocked. People die every year through dangerous fumes backing up inside their car.
5. In a heavy snowstorm, tie a colored cloth to your antenna or door
6. Exercise to keep warm and keep your blood flowing - People have survived being trapped in a walk-in fridge that way
7. If stuck firmly in a traffic jam, get to know people around you and check on each other for support.
Many guides recommend to raise the hood after the snow stops falling, and leaving the light to indicate distress, but I don't think that's normally smart.
Warning signs for hypothermia include shivering, memory loss, disorientation, incoherence (potentially including exuberance), slurred speech, drowsiness.
Frostbite causes a loss of feeling and a white or pale appearance in extremities, such as fingers, toes, ear lobes or the tip of the nose.
We should probably have a separate topic for that.