Okay, so mine doesn't look all Martha and everything, but I reuse a shopping bag and designate it as a collector of donations. As we identify clothes, shoes, or other items that we no longer need/want, I put it in the bag. When it's full, I schedule a pickup by the Vietnam Veteran's Association. Details are in this post. They come to your house, pick it up, and leave a form to be filed away for next year's taxes. (Bonus points: write down your estimated value before you file.) Couldn't be easier! (There are lots of other places to give - I believe Red Cross will also pick up - and of course there's your local shelter or goodwill.)
The key is to designate a place for these items and to do it as you come across things you don't need any longer, thus avoiding the "big purge" we all tend to do once or twice a year.