Keep your eye trained on pieces that play an important role in history, relate to a famous person's legacy, or tell you something interesting or personal about a historical event.
Examle: while many documents signed by Lincoln relate to promotions, and these are desireable, this letter relates to a policy of great import during the war and represents a fascinating moment during the war. Additionally, it shows Lincoln's involvement in matters not only of military tactics but of strategy.
Nathan Raab
Nathan Raab is Vice President of The Raab Collection and on the Board of Directors at the Historical Society of Pennsylvania. He is a regular contributor to Forbes.com.