Tabards are actually supposed to flap around. Watch your character when you play next. The tabard is free flowing, for the most part. You can secure it at the waist with a belt, and it won't be as....flappy? Sewing the tabard onto the costume would just make it part of the Druidic robe, not a tabard itself.
I myself play a Human Priest on Stormrage and a Worgen Rogue on Kilrogg with many alts spread across the realms of Azeroth.
[i]Kirito (Aincrad, not ALO)
Ino Yamanaka 75%
(Need wig cut and styled)