Originally Posted by FBinNY

Before sweating how to mount a flag, consider what you're trying to achieve.

If you're worried about improving visibility to drivers approaching from behind think about which way the flag flies. As you ride, the flag will tend to point straight back, presenting a minimum (edgewise) profile to drivers directly behind you.

Of course, cross winds will help fix that, and the flag is also very visible to traffic abeam of you, but otherwise it's of extremely limited benefit.