Odds of Getting In
Getting a job in history is a lot harder than getting a passing grade in history class. History is one of those disciplines that's fairly easy to understand (stuff happened), so it might seem like a more sustainable career path than becoming a biochemical engineer or a theoretical mathematician.
But with competition so fierce, your odds are best helped by getting the same level of training as any of those other math-and-science gigs. The more training you have, the better your chances.
Like other careers, it also helps to network. You can network your brains out by joining any of the hundreds of associations, societies, and organizations dedicated to the study of history.
From the American Historical Association to the National Railway Historical Society to the Medieval Academy of America, there's a club out there for historians of every interest. Members of these organizations benefit from networking with other members and obtaining access to job boards and grant directories.
It's about who you know just as much as it's about what you know.