I for one am a huge fan of organizing and down sizing... it is the best kind of clean. And one of the best places to do both of those things is in the closet. While swapping out your winter wardrobe for spring/summer clothing, take some extra time to look for items you can donate. Everyone has clothes they keep and never wear so ask yourself this question: "if I saw this today in a store would I buy it?" If the answer is no, put it in the donate pile, chances are if you wouldn't buy it... you won't wear it either.
Once you've filled a few shopping bags decide on an organization to bring your donations. Or you can have them come to you. I regularly request pick ups from the Military Order of the Purple Heart. They are a congressionally chartered veteran's service organization and will accept donations of many kinds from clothing to electronics. They'll even schedule a pick up day with you and send a truck to collect your donation... how much easier could it be?
Last time I cleaned the closet Jared even joined in and between the two of us we must have donated eight large shopping bags of clothing. Most of it was really nice clothing that we just don't wear anymore (or in my case, all of my old shirts that were super short... exposed belly, not so much).
If scheduling a pickup isn't your thing, try looking for drop boxes in your area. Salvation Army has these set up in various parking lots and accepts shoes and clothing. It can simply be an extra stop on your way to the grocery store.
Another organization I have found success with is Habitat for Humanity. They will collect larger items like furniture. I discovered that some places do not accept furniture anymore but Habitat will gladly send a truck to pick up most pieces for their ReStores, which are retails stores that sell anything from furniture to appliances. Last week I had them pick up the "monster"/TV armoire that I have been itching to ditch for months. It's finally gone and we are on our way to redecorating our room off of the kitchen.
One last place I'll mention for taking donations (or recylcing old stuff) is Home Depot. Last time I was at HD I noticed a tall drop orange box near the entrance. These boxes are for two things; recycling old electronics like cell phones, and disposing of CFL bulbs (energy saving light bulbs should not be thrown out with household trash as they can release harmful toxins into landfills.) Technically this isn't really considered "donating" but it is helping the environment and if you're cleaning out closets you are bound to find stuff that needs to find a new home.
Don't forget to ask for a receipt after you make a donation. Save it in a folder for tax purposes next year. You'd be surprised how much you get back for donations. Helping yourself and helping others... it's a wonderful thing.