I'm sure that if you haven't already tried one of these, you've at least heard of them. Naked Lady Party, Clothing Swap, Buffalo Exchange: it's an event of many titles, but the general gist is completely simple. Get a group of girls in a room with their closet's cast offs, pile it all in the center of the room, and dive in!
Some people like to make it a more organized event, with rolling racks and designated piles for sweaters, jeans, etc- some even separate by size. I've heard of people making special invitations, too. We've done our fair share of these over the past few years, and we've found that a simple evite and a giant pile in the center of the room does us just fine. I would universally recommend a full-length mirror, though.
(there I am!)
Daunting as the room-sized pile may seem, after everyone's finished extracting their new (free!) treasures, it's surprising how easily and quickly the leftovers get bagged up and brought to the nearest Goodwill.
It's a pretty fantastic and satisfying Sunday afternoon activity. Make it happen!