Narrative
Make sure the chronology is always clear. The reader should never feel confused about when things are happening or in what order. You can go out of order if it helps make better sense of the story, but you need to let the reader know what you are doing. USE DATES.
The beginning is important. Your intro paragraph should get the reader oriented (Who? What? Where? and most importantly When?--think hard about where this story should begin) and set up some tension that the rest of the narrative will resolve.
Think carefully about what to include in your narrative, and more importantly, what to leave out. Don't distract your readers with unnecessary details. They will lose track of what you are trying to show.
Go beyond presenting a mere list of one event after another. A narrative needs to analyze and explain. Show development, illustrate connections between events and cause and effect, develop an overarching theme, identify key turning points, build to a climax.
Think hard about when to end the story. If you are writing about the Revolution, don't end near the beginning of the story, in 1765, or at a key turning point, 1776. The Revolution lasts at least right up through the Peace treaty with Great Britain. Some would argue it continues until the Constitution is ratified.
Argument
Always state your argument quickly and concisely, as early as possible in the paper (in the first paragraph, that is, but not usually in the first sentence--it's better to explain the question first).
When you make an assertion essential to your case, provide some examples as evidence.
Give the fairest possible treatment to those against whom you may be arguing.
Always admit weaknesses in your argument and acknowledge those facts that opponents might raise against your position. If you deny obvious truths about the subject of your argument, knowledgeable readers will see what you are doing and will lose confidence in your sense of fairness. Most arguments have a weak point somewhere. . . If you admit the places where your argument is weak and consider counterarguments fairly, giving your reasons for rejecting them, you will build confidence in your judgments among readers.
Stay on the subject throughout your essay so your argument is not submerged in meaningless detail
(Taken directly from Richard Marius, A Short Guide to Writing About History, 3d ed. pp. 68-70).
Exposition
"Expositions explain and analyze--philosophical ideas, causes of events, the significance of decisions, the motives of participants, the working of an organization, the ideology of a political party. Any time you set out to explain cause and effect or the meaning of an event or an idea, you write in the expository mode. . . . The exposition . . . defines terms, and provides context. . . . [it] includes some inferences. . . . One cannot prove an inference, but inferences provide plausible explanations that may help fill out the meaning of a text. These inferences show a writer trying to make sense of things, and readers appreciate such thinking as long as it says within the boundaries of the plausible" (Richard Marius, A Short Guide to Writing About History, 3d ed. pp. 61-65).
Like other historical essays, the exposition needs to being with an introductory paragraph that orients the reader in time and space and explains the purpose of the essay. You may also articulate here a broad thesis, or statement of the significance or meaning of the topic you are writing about.
When writing an expository essay, it is important that you provide full explanations of the various components of the topic ("develop") and provide links between the various elements of it. Use paragraphs to highlight important divisions or subtopics, but provide appropriate transitions or links between these paragraphs. While full development is important, a certain amount of simplification and systematizing of complex matters is necessary for helping the reader to understand. This often means leaving out some information that may not be essential. You also provide interpretation for the reader--explain the meaning of something, like a historical text or an event, that is not apparent on the surface. Why did the inhabitants of a city riot against the government? What was the meaning of the Second Amendment of the Constitution?
Description
"Description presents an account of sensory experience--the way things look, feel, taste, sound, and smell." Try to include "concrete details about physical reality" in your writing to capture a vivid sense of what the past was like. "Never try to describe everything. You will suffocate your essay in details. Describe only enough to kindle the imaginations of readers. . . .
"Another kind of description here is more impressionistic, more metaphorical. Russell tells us that the enclosure where prisoners sat in the courtroom had 'nothing formidable or forbidding about it.' He tells us that the sheriff carried an official staff 'that he weilded like a benevolent shephard.' These are his impressions. . .
"Never make things up when you describe something. Although some readers may be intertained by flights of fancy in historical writing, historians find them cheap and dishonest, and with good reason."
(From Richard Marius, A Short Guide to Writing About History, 3d ed. pp. 50-55)