I have used many devices over the years, since I commuted for about 20 years and rode a lot at night. I started with white bicycles, then tried 3M's glass bead reflective paint, which looked great under headlights but was dull otherwise. I am now using neon orange bikes, and may go to neon lime green. I think all of those approaches have helped over a standard dark frame, since seeing a frame identifies the vehicle immediately as a bicycle. Unfortunately, all reflective products rely on being in the beam of a headlight, so for a lot of situations they are not much help.
Flags are great.
I use one or two on my recumbent. One has a blinking white strobe light on the top of the shaft. The blink of a strobe disappears too fast for the eye to follow it well, but combined with the flag it's better, and it gives a 360 degree flash. For headlights I use a car light. Nothing makes a driver respond quicker, even in the midst of urban light clutter, since they are conditioned to look for oncoming cars, and that means they are looking for oncoming car lights.