I like Chuck Taggart’s idea, if we’re going to be flag-wavers, of waving a Whole Earth flag (left), which I’ve always found to be awe-inspiring. Looka!

Taggart also points to this Earth Flag (right), which flies over various SETI Project sites. It is more stylized (that’s the sun, the earth, and the moon, if you hadn’t noticed) and, I think, equally inspiring in its way. I’ve written to see if these are available; if anyone’s interested, I’ll let you know.