My cardinal rule: Stop 'splaining so much. Jane Austen didn't interrupt Persuasion with a disquisition on the Napoleonic Wars. Don't you do it either. The master of historical fiction is Patrick O'Brian, whose yarns are indeed ripping (see below), and who doesn't clog up the works with a manual on naval battle terminology. (Although his readers do.)
I first noticed the amplification phenomenon when, in the midst of reading my way through the O'Brian series, I picked up a copy of one of Steven Saylor's books, which are set in ancient Rome. I'd read a few before and enjoyed them. I believe the author stopped in the middle to explain what fasces were. And it pissed me off since a) I already knew that; and b) it ruined the mood. BTW, Saylor also subscribes to Rule VIII (see Rules for Classical-Set Fiction below): "Your Hero must find slavery, crucifixion and gladiatorial combat Morally Repugnant."
Another writer who doesn't commit the cardinal sin is Harold Adams, whose Carl Wilcox books are set in Depression-era South Dakota.
Anyway, here are the rules that started me on this:
Miriam Elizabeth Burstein's Rules for Writing Neo-Voctorian Novels, which linked to:
Alan Fisk's Rules for Writing Historical Fiction/Writing Ripping Yarns, and
Susan Higginbotham's Ten More Rules for Writing Historical Fiction, and
India Edghill's How To Write Feministly-Reimagined Historical Novels, and
Elizabeth Chadwick's The Official Rules for Writing Medieval Fiction.
See also Rules for Classical-Set Fiction.