2 lights:

PDW DangerZone - steady (on seatpost)
PDW Radbot 1000 - flashing (on rear rack)

I like one flashing light to get attention, plus one steady light to help with distance and velocity perception.

Plus reflector (comes as part of Radbot)
Plus retroreflective tape on fender, bike, and helmet
Plus retroreflective panels that come standard on Ortlieb panniers


I like the suggestion of putting a light on the helmet, to get it higher off the ground and also to get more separation from other light. Have not tried that yet, though.