We used to call them naked lady parties, although no one ever got naked. Get a few friends and friends of friends together, bring a bag of clothing that you've outgrown, shape-wise or style-wise, hang out, and exchange the goods. One lady's cast-offs are another's new score. Nothing bought, nothing wasted, donate the leftovers to the nearest Goodwill. (The downside of swapping within the same circle was that you might see a shirt you gave to a friend at the last naked lady party rear its ugly head at the next one when the recipient decided she didn't like it that much.)
In recent years such clothing swaps have branched out to become public events, and not just for ladies. ThriftOn, held at a downtown Manhattan bar is a chaotic free-for-all drawing more than a hundred swappers. Swap-O-Rama-Rama, which occurs periodically in cities across the country, even provides to modify your clothes on the spot. You can also find a clothing swap near you via MeetUp.com.
There are rumors of more "exclusive" swaps asking for a 30-dollar entrance fee to even attend, which seems to fly in the face of the freegan-esque, just-among-friends mentality of the more homegrown swaps. Then again, I never scored a pair of Prada pants at any of the naked lady parties I've attended.