Key tip: In fog, turn on your headlights, not your park lights, and don't rely on the Auto Headlight setting
You should also:
Slow down to suit the visibility levels.
Keep your headlamps on low (dip) beam, as high beam will reflect off fog and produce glare, further reducing visibility.
Keep a generous following distance between you and the car in front.
Remember that automatic headlights in cars can be fooled on foggy days. There is enough light to make the car think it's daylight, and switch the lights off. Even when visibility is poor due to fog. Switch them to "On". Don't just rely on the "Auto" setting.