Arrange your books by category, such as educational/self-help, fiction and non-fiction. Many organizations accept donations of specific types of books. For example, Books for Africa.org is seeking law books to help people in Africa who are trying to get a law degree. Knowing the kinds of books you have will help you find the appropriate organization.
Ask your local library if they are working with any charities. Many libraries partner with local charity groups that work to get books into other libraries that can't afford to purchase many books.
Locate a donation drop-off location in your area. An Internet search will yield several organizations where books can be donated. For example, Gotbooks.com offers locations across the country.