If you are pursuing, or interested in pursuing, a Ph.D. in history with an eye to an academic career, these two pieces of advice will help you.
(1) Start looking at the job listings in the Chronicle of Higher Education sooner than you think you need to. Jobs in your area of history will invariably include something to the effect that a minor field in such-and-such area is a plus. Thus many ads for U.S. historians will say “background in Latin American history [or whatever, though Latin American is very common] is preferred.” While you still have coursework to complete, take courses in whatever outside field pops up in the most job descriptions. No one has these outside fields, I assure you. Job applicants don’t realize they need them until they’re reading job descriptions and it’s too late. In an extremely competitive academic job market you will stand out if you actually have the additional background the hiring committee wants.
(2) Subordinate all other writing and projects to the overriding task of getting an article published in a highly regarded scholarly journal. Seek out conferences where you might be able to present a paper dealing with your topic of research. (Keep abreast of upcoming conferences here.) If you can get one or even two scholarly articles under your belt by the time you go on the job market, you will stand out. Very few applicants will have this. The process of getting an article read, commented on, and accepted is often quite time-consuming, so start in on this as soon as you can. If you have paper assignments for your graduate courses, see if you can kill two birds with one stone by covering a serious topic that can one day become an article.