As the internet increasingly makes encyclopedias obsolete, there are still many places around the world that could really use them. Schools all over the world are desperately in need of quality books--especially reference materials. Consider donating the dusty set of encyclopedias you have sitting on your shelf to someone who will actively use--and appreciate--them.
Perform an internet search for information on international book donations. There are a number of organisations that ship books overseas. However, the criteria for acceptable books varies by organisation. Be sure that the organisation you choose will accept encyclopedias. The American Library Association (ALA) has a list of international donation organisations on its website.
Assess the quality and condition of your encyclopedia. Damaged or mouldy books are of no use to anyone. The ALA website lists some guidelines for international donations: "[M]ost organizations only accept new books or books in good condition--these are not places to 'book dump' unwanted literature. Books that are outdated, damaged and worthless are going to be just as useless abroad. Make sure that the information is going to be useful to the schools or people receiving them."
Pack your encyclopedia into strong boxes. It's usually a good idea to split them up into two or three smaller boxes, rather than one large box, depending the size of the collection.
Address the boxes properly and take them to your local branch of the U.S. Postal Service.