Below is the advice that Michael Golay, Exeter history teacher and prolific author (here is a link to his Amazon page), gives to his students.
Every word, every cadence, every detail, should perform a definite function in producing an intense effect. Samuel Taylor Coleridge
• Start with lightning … Your opening sentence should catch the reader's attention. Your opening paragraph should suggest the main themes you intend to develop. If it's an anecdote (story), make certain the story has a thematic point.
• The last sentence of each paragraph is critical – it is the launch pad to the next paragraph. Don't repeat what you've already written; your last words should be a transition to what's to come.
• Take care that your verbs are vivid, active; they are the engines that drive your sentences. Do everything you can to avoid the passive voice.
• Bring your prose to life with carefully chosen quotations from primary sources; these add voices other than your own. Quote in short bursts, not in long blocks – your object is to give the effect of reported speech. And be sure to attribute quotes: "You have free liberty to go away from us," the Puritan magistrate John Winthrop reminded dissenters.
• Report the details that matter. Sometimes the weather is just the weather. In other cases – a battle, for example – whether it's raining or sunny, hot or cold, can determine the outcome.
• Be conscious of cadence. A string of simple sentences becomes monotonous. Variation is the key; remember, too, that an occasional short sentence (perhaps set off as its own paragraph) packs a powerful punch – will stay in the reader's mind. Read aloud to make certain your prose is pleasing to the ear.
• Avoid insipid modifiers such as very.
• You've heard this in English – show, don't tell. You probably will have a discrete "thesis statement." Alternatively, you might want to let your analytical points "bubble up" from the narrative. In writing history, we show to tell.
• Be mindful of context. Events don't happen in a vacuum. And keep the reader oriented in time – don’t make him guess what century you're writing about.
• Craft a stirring conclusion, one that echoes but does not simply repeat your argument or line of interpretation.
Sources: Strunk & White, The Elements of Style; Zinsser, On Writing Well; Graves and Hodges, The Reader over Your Shoulder