List of communities to involve:
bc
gc
voa
lbl
bl.net
bm

Originally stolen from Daneel, slightly modified:
1.  The flag should appeal to BLs and GLs, and also to our young friends (boys and girls, younger and older) for both colours and symbols, to maximize acceptance by the community.  This would rule out, for example, the overuse of pink (and some might say any use at all of pink, though I'm not entirely sure I agree)
2.  The flag should be easy to manufacture, to maximize the chances of actual use.  You shouldn't have to go to a world expert flag manufacturer to make one, but rather someone with some cloth and a sewing machine should be quite capable of making their own.  This would rule out, for example, detailed drawings, text, fancy 'shields', and so on.  Stick to the usual standards of flag design.
3.  The flag should be simple, its symbols striking, and its symbolism meaningful.  Perhaps the best example of this would be the Canadian flag:  only two colours, but the maple leaf is striking and grabs the viewer, and the Canadian maple was already a strong symbol of Canada, and the sea-to-sea of the red-white-red.  Another great example is the GBLT Rainbow Flag, which is visually gorgeous, and simply symbolizes diversity.
4.  Equality, fairness, balance.  Every effort should be made so that any symbols or colours related to BLs or GLs, girls or boys, portray them equally and fairly, i.e. balanced.
5.  Flexibility. It might be nice if the flag allowed some modifications to it, like the changing or adding of a symbol for different occasions.  Another example might be a flag which stands for CL, but which could be adjusted slightly so as to be used exclusively by BLs, or exclusively by GLs, if desired, perhaps by changing all pink to blue, or vice versa.