Low beams, not high beams
Fog reflects high-beam light back at the driver, making the road harder to see. Use low beams or fog lights only. Many cars also have rear fog lamps that make you visible to drivers behind you — use them in heavy fog and turn them off when conditions clear.
Slow down to a speed you can recover from
Drive slow enough to stop within the distance you can see. If your visibility is 100 feet, your safe speed is well below 30 mph. Use the right edge of the lane as a steering reference; the white line is much easier to follow than the dashed center line in fog.
When to pull over
If you cannot see the lane lines or the car ahead at all, pull as far off the road as safely possible, turn off your headlights, turn on your hazards, and wait. Headlights left on while parked can be mistaken for an active vehicle and cause rear-end collisions.