Just like in "real life," the goal of good history writing is to persuade your reader of a main point.  You will use similar strategies both when persuading your boss in a written report that you have a good idea and when persuading your history professor that your interpretation is sound.

Prepare well.  Write an outline, which will insure that your ideas are clear before you start writing.  Include in your outline a one-sentence statement of your main argument.  Make sure that this thesis statement is an interpretation and not just a fact.  "George Washington was the first president" is not a compelling thesis, but this is: "George Washington was a leader who paid more attention to power politics than daily governance."  Then write a rough draft based on your outline.

Write clearly.  Since your goal is persuasion, you need to make sure your writing is as clear and powerful as possible.  Metaphors may be more appropriate for poetry:  while they sometimes can make your point well, often they are vague.  Sentences should be simple and straightforward.  There is no need for very long sentences with multiple dependent clauses.

Organize your writing.  Because your reader may be reading fifty other papers you need to put your arguments in a clear and organized format.  Your paper should have an introduction in which your thesis is clearly stated.  Each paragraph should contain one main idea, stated in a topic sentence, that is supported with evidence in the body of the paragraph.  Make sure you have a conclusion that sums it all up.

Use specific evidence.  Each paragraph should contain one or two pieces of evidence to support the topic sentence of that paragraph.  If you quote from a book or article, don't use a quote that is too long.  This makes your own voice disappear from the paper.

Footnote.  It is plagiarism to use ideas from another published source--in print or on the internet--without acknowledging that source in a footnote.  Of course, this does not apply to simple facts, such as "George Washington was inaugurated in 1789."  But any interpretation or quote from another source must be footnoted.  The standard style includes author, title, publishing place and date, and page, like so:  Neal Salisbury, Manitou and Providence (New York, 1982), 67.  If you prefer, you may use the parenthetical style:  (Salisbury, 67).  In this case, include a bibliography at the end of the paper that has the full citations of the books and articles used.

Think critically.  How could someone criticize your thesis?  How would you defend your thesis against this criticism?  You can make this process explicit, including a paragraph that acknowledges potential criticisms of your thesis while showing how your thesis actually stands up to those criticisms.

Proofread.  Pay attention to spelling and grammar.  Careless mistakes make the reader think you don't care.  Read your paper aloud to see if the sentences flow.  Give a rough draft to a friend or roommate and ask for criticisms and advice.