This.

Also, if you use a saddle bag, you can put a taillight on the back of that. Many saddle bags come with a taillight loop preinstalled. If not, you can go to the fabric store, buy some webbing, thread, and a needle, and stitch one on yourself. I hook a taillight onto the saddle bag, and then use a glue gun to glue the hook and the back of the taillight together so that it can't bounce off.

Also also, for redundancy, you can bungee a light to the back of your helmet.

Also also,