Read as many historical novels as you can to get the rhythm of the genre down pat. After a few books, you'll start to recognize conventions used over and over again in the historical novel genre. Also read books on writing such as "Plot" by Ansen Dibell and "How to Write the Breakout Novel" by Donald Maass. Donald Maass is an agent for historical fiction novelists as well as a writer.
Avoid forcing modern people into clothes from days gone by. Many writers use forget that as the years pass, attitudes about certain aspects of life change. Study more than the dates that things happened. Understand how your characters' minds worked and why they thought, spoke and acted the way that they did. For people who actually existed, don't just rely on the popular myths about them. Spend time in research libraries that will have letters they might have written or newspaper clippings that aren't widely-known or published in textbooks.
Sweat the small stuff. Details are as important in historical novels as they are in mystery novels and maybe even more so. The details are what helps paint the picture and get the reader feeling transported back in time. Head coverings, slang and even furniture all help to make the day-to-day setting of your story feel more authentic. Visit a museum with an exhibit dedicated to the era so that you can see paintings that would have hung in these people's homes or pottery they may have used in an ancient civilization.
Before you complete the novel, visit the place. See the ruins. Study the architecture in the buildings. Find a list of historical landmarks from the city's visitor's center and visit the places that were from the era you're writing about. Try to channel the energy of the people who inhabited those rooms and walked those halls.