For those of you in San Francisco, Roman Mars did a TED Talk about their flag design and specifically levels good critiques against the San Francisco flag, lays out ways to make it better, and shows examples of good flags along the way.
http://99percentinvisible.org/news/99-invisible-at-ted/The precursor to that talk was an episode on flag design: http://99percentinvisible.org/episode/vexillonaire/