There are yellow bins for clothing donations, in some parking lots around the neighborhood … don’t recall where, but if I remember I’ll re-post.
But this may be even easier: does a monthly pickup run throughout DC — which day depends on where you live. If you have something for them to pick up, you let them know via phone or email, and just leave it in boxes labeled Purple Heart at your front door by 7am on pickup day. They collect it all and leave you a receipt. Their site has more info on what they’ll take, but it includes clothing, household appliances, etc.
For contact info, to call for their schedule: http://purpleheart.gooddonor.org/web/pagedetail.aspx?pgID=contactus
What they accept: http://gd.purpleheartpickup.org/gd.donations/main/acceptabledonations.aspx