I mounted my rear light on the lefthand seat stay. That way as long as I don't ride in the UK, Down Under or any of those other places with backward roads I'll always have a light on the traffic side of my bike. Just for a Zen-like symmetry, I mounted the little red reflector I had on the other seatstay.

Actually, my seat wedge has a loop for a light, but it was impossible to get it to actually shine outward instead of down at the wheel.