1. Fashion Project
You ship your almost new or gently worn items and get a free gift card! The shipping is paid by the Fashion Project. They sell all the clothes on their website and 55% of the profit goes to your chosen charity. So, you kind of win twice - you donate to a favorite charity AND get a gift card (right now it's Lord&Taylor's).
2. Lean Closet Movement
You ship your stuff (for free) to the Cuyana shop who's motor is "fewer, better things" and get $10 discount towards your next purchase with them.
Your drop whatever clothes you have (even seriously worn or old) in your own bag (prepare to part with the bag) at any H&M location (usually near the cashier) and get various discounts for your next H&M purchase.
And there is always a secondhand, of course, but I'm not a big fan, to be honest. There is something about people going through my stuff and judging it that deeply bothers me.