Be objective. While everyone holds some sort of bias, good historians do not let bias cloud their research or conclusions. Be sure to acknowledge and discuss differing perspectives other than the particular argument your research supports.
Stick to the past tense as much as possible. Do not write about long-past events and long-dead people in the present tense. (Courtesy of Boston College History Department)
Avoid contractions in your academic writing. For example, use cannot instead of can't, will not instead of won't, do not instead of don't, etc. This makes your writing more formal and professional.
When writing an historical paper, remember that each body paragraph should focus on one idea or aspect of supporting your argument. For example, if discussing the women's suffrage movement in the United States, a body paragraph might focus on the Seneca Falls Convention using the Declaration of Sentiments as a piece of primary source evidence. This paragraph should introduce the paragraph's topic, provide evidence, provide a sentence analyzing that evidence, and then link that paragraph's arguement back to your thesis. Remember MEAL--Main Ideas, Evidence, Analysis, Link!
When in doubt, cite it! Remember that you must cite ideas that are not yours, not just direct quotations. If something is not considered "textbook knowledge," you must cite it. For example, "Rev. Martin Luther King Jr. was a leader during the Civil Rights Movement" is considered textbook knowledge. You cannot reword someone else's ideas and claim them as your own. If you are unsure about whether or not your should cite something, go ahead and cite it!
Avoid generalizations in your writing. Be precise and try not to assume things about large groups of people. For example, avoid statements like, "women in the 1920s became flappers who believed in liberating females." Not all women were flappers or approved of their lifestyle. A more precise statement might be, "many young women in urban centers adopted the flapper lifestyle with the goal of female liberation." This statement is true, and avoids classifying all women in the 1920s as flappers.
Use active voice in your historical writing, for example, "Van Gogh painted Starry Night." Avoid passive voice, for example, "Starry Night was painted by Van Gogh." When writing, it is important that you are clear and concise about who is performing the action of your sentence, and that the subject of your sentence comes before the verb.
When referring to an historical actor, Benjamin Franklin, for example, never refer to them by first name alone. Introduce the historical actor by using their full name (Benjamin Franklin), and then refer to the historical actor by last name thereafter (Franklin). NEVER refer to someone by first name only (Benjamin), as you would a personal aquaintance. This is the best way to maintain respect and professionalism in your writing. Judges will look for this during the contests.