Don't drift. There's a natural tendency to wander into the middle of the road when visibility gets bad. Be sure to keep yourself in your lane.
Beware of critters. Animals--especially white tail deer--feel bolder under cover of fog and are much harder to see.
Beware of Freezing Fog. In some climates, fog near the freezing point can freeze on contact with cold surfaces--including roads! This can cause black ice.