A Geeky Button
What's the fastest way to identify fellow dorks/potential friends? WHY, by showing off an awesomely geeky button, of course! Send a subtle signal to other members of Dumbledore's Army by sticking one of these babies (from Zazzle.com) to your backpack, and you'll have a boatload of buddies in no time. Bonus: All of them will be really, really good at defeating Dementors.