W.P.A. Poster, 1938, from the Library of Congress. There are thousands more like this!
Historians work in several ways to analyze the past -- some of these approaches are more suited to a question or document than others, and sometimes historians combine these approaches. For the most part, historians look for patterns that explain the past, and you can reveal these patterns with different approaches:
1. Change over time - the history classic. This approach uses a sampling approach to study how a specific topic or attitude changed by sampling documents from a specific starting point through the moment of specific change:
How did attitudes about the Vietnam War change?
This paper looked at newspaper accounts of the war, editorials, and the
reports of protests at three moments: prior to US involvement, at the
start of US involvement, and at the height of popular protest.
2. Comparing point of view - another classic. This approach works with the truth that historians hold very dear: everyone has a bias! Historians begin with a subject that is controversial and basically studies the controversy:
How did contemporaries view President Buchanan's efforts to prevent the Civil War?
This paper looked at three points of view: southern newspapers,
and the papers of President Buchanan himself.
The study was limited
to the six months leading up to the war, and
had a huge amount of material to work with.
3. Themes at work - or, use what you learned in your English class. In this approach, historians work with a group of similar documents -- posters, photographs, advertisements, etc. -- and looks for patterns, and then tries to find meaning in those patterns. As you can see, this approach is especially suited to visual documents.
How did the WPA posters of the 1930s persuade Americans to live healthier lives?
This paper looked at a hundred of these government-sponsored posters which
promoted everything from vaccinations to the importance of washing your hands.
The paper looked at the pictures and copy [words] in the advertisements, and found
that they used a mixture of fear, encouragement, and simple information to get
Americans to straighten up and fly right.
4. Revealing the obvious in plain view. Seriously. What historians sometimes do is analyze an event which is in plain view, but has not been considered historically. It might be fashion, it might be legislation, it might be education reform:
How did the People with Disabilities Act of 1976 demonstrate the activism of the disabled community?
This paper looked at the specifics of the Act as well as the speeches and interviews with the
activists and categorized their goals: specific aspects of federal legislation, changes in peoples'
attitudes, and the role private business might play in getting the workplace to accommodate the disabled.
5. Revealing the obvious with a magnifying glass. In this case, historians begin with an already-known truth, and through a more careful analysis of the evidence, attempt to give a fuller explanation of events.
Given American neutrality in the 1930s, how did the American public view Hitler's control of Germany?
This paper combined the "change over time" with this idea of revelation: it took as its
starting point that America was heavily neutral in the 1930s, and framed a time period of 1933
[when Hitler became Chancellor of Germany] and 1941 when the US declared war on Germany.
6. Revealing what's obvious now, but wasn't then. This is the most Sherlock Holmes-ian we historians get to be -- revealing the truth about something that everyone thinks they know about:
What did Americans of the 1950s and 1960s know about the CIA's role in controlling governments in Latin America?
This paper started out with what we know now -- that the CIA actively worked
to control elections in Latin America to make sure that these governments
were "friendly" to American interests. The paper examined newspaper articles
and editorials from the 1950s and 1960s to see how much of this activity was
revealed in the press, and how it was portrayed.
 The professional organization for historians is the AHA, or the American Historical Association. There is also an honor society for college history majors; the induction ceremony is secret, and involves white candles and the swearing of oaths. Then you get to eat lasagna.