In addition to what's been said about the reflectors, I find it handy to have more than one tail light, just in case one DOES run out of batteries. Headlights (which don't seem to have been discussed in this thread) are also important if you want to avoid pot holes in the road, or if you want to be seen by crossing traffic at intersections. I currently run an E6 headlight powered by a Schmidt hub dymano as a primary headlight, with a 5-LED battery powered back up mounted on my handlebars. The LED light also doubles as a camp flash light.

Once you have the right lighting equipment, you may come to the realisation that riding at night is actually SAFER than riding during daylight. I know I feel that way.