Lights day and night! I like to be seen and see as much as possible. Believe it or not, I actually feel safer riding in the dark of night around here. I feel I can be seen with 2 red strobe lights from behind much more that during the day. If I am near the shoulder at night on the other side of a parked car, I'm pretty sure a driver coming from behind will know I am there. During the day, it's a gamble. The strobe light can't be denied.
I also like ankle bands in the dark to be seen from the sides. Heck they even light up with the camera flash.
2 red strobes on the rear, one on the helmet and one on the seat post. Even lights during the day light.
I even put the lights on our bikes during the day, that's Gina.