Once a month (or so) we go through our closets and drawers and pull out the stuff we've stopped wearing. We fill a bag and take it to the local thrift store. Often, the clothes in those bags have barely been worn. Now, instead of giving away those gems, you could swap them for other pieces of gently-worn clothes ...
ThredUP is a pretty interesting concept. It works much like the book-swapping sites we've talked about here at Re-nest (Bookmooch, etc).
You create a profile that lists your size and describes your style, you list the items of clothes you're no longer wearing and are willing to swap, and then you send and receive items of clothing using thredUPs prepaid envelopes.
It sounds like a pretty sweet setup. We're definitely going to give it a try—we can already think of a few items that we'll be sending off very soon.