When I used to commute, I used a cobbled together plastic mount for mounting a cygolite hotshot way down low on the seat stay and then the same light on the back of the seatpost (2 lights). For riding in the dark, I set the low light to an annoying flash mode to call attention to my self, and set the higher light to steady on, this is to give motorists a way to estimate the distance and rate of closure etc. ( I read somewhere that with a flashing light, motorists cannot determine distances). this strategy works only if the lights are separated by a certain minimum distance (I think 1.5 feet). any less and the lights merge into one visually.
Also, studies have shown that the up and down movement of your feet has some special power to be noticed because we are programmed to see biological movements. I have been playing with a way to get light on each of me heels to take advantage this. if I ever start riding in the dark again, I will play with this,