Bright, bright coloured clothing ... hi-vis yellow or orange jersey or jacket preferably with reflective stripes.

Rear reflector, reflective tape on your mudguards, reflective ankle bands, bright coloured reflective vest.

You do not want to blend in with the fog, so do not wear dull or dark colours.

I would probably add one or two more red blinkie lights in the rear, and I would probably attach a bright white or amber blinkie in front.

And if the fog was really thick, I might be tempted to ride (very cautiously) on the sidewalk. I would NOT normally recommend riding on the sidewalk ... it's illegal in many places, and generally quite dangerous. But in a situation like really thick fog, I might feel the sidewalk was safer. If I rode on the sidewalk, I'd keep my pace down, and would stop, check for any traffic, and walk across every intersection. In other words, I would behave like a pedestrian.