This one is so easy. If you're stumped about what to do with your old cellphone(s), click on this logo and fill out the form. The Good Deed Foundation will mail you a postage-paid envelope. Stick your phone(s) in and mail it off. The phones are recycled around the world. Piece of cake.
There are variations on this program that send phones to soldiers, local food banks, and other charities. ReCellular is a good overview of different causes you can support with your old phone.
If you must be a selfish bastard, there are also companies which will pay you for your phone. You know, if you can't figure out eBay.