In addition to lots of reflective material and a good headlight, I recommend the super-bright Dynotte 400R tail-lightDynotte 400R tail-light.
This light is the brightest I could find commercially available and seems to be visible from more than 1/2 mile away in bright sunshine (and much further at night). I ride with it on at all times of day while commuting year round.
In my side-by side tests the 400R was significantly brighter than my car's break-lights. In the past 8 months I've had about 10 comments from drivers and as many from other cyclists about how bright the light was.