Ted Kaye has compiled some advice for designing flags.
1. Keep it simple.
2. Use meaningful symbolism
3. Use 2-3 basic colors
4. No lettering or seals
5. Be distinctive or be related
In a nutshell:
The flag should be so simple that a child can draw it from memory.
The best flag in the world follows all of these rules.

That's the flag of the Benin Empire, a pre-colonial African state situated in modern Nigeria that lasted from 1440 until 1897. (via andre)
Pie charts representing the flags of the world's nations...the area of each color on the charts corresponds to the percentage of that color used in the respective flag. I'll take this opportunity to again maintain that Rem Koolhaas' barcode flag for the EU is, technically speaking, wicked awesome. (via colourlovers)