I'd recommend one of the Amphipod Xinglet jogging cross belts. It's the same kind of reflective harness/belts that police wear for motorcycle/traffic directing duty. Very lightweight, reflectively, hi-viz lime green straps, fits over any clothing, and best of all folding into something as small as a knit cap. I hear the flashing version is quite bright, but that the watch batteries the active flashers use burn out quickly. I personally just use the standard hi-viz reflective one.

If you have a set of fenders you might consider adding a reflective stripe or 12 square inch "cotton tail" as once required by British/Dutch law on bicycles for traffic visibility.