Immersion. I think there is much to be said about immersing yourself in one period and knowing it inside out. Your writing just flows and all the little details come out naturally without it being an info dump.
Antebellum/Civil War South happens to be my specialty. I've been obsessed with it since I was a child. My focus is generally social history and women's history. Something very important to consider while researching this period is bias. A lot of researchers really harp on the slavery thing, and as such, don't give the south a fair shake. Catherine Clinton writes excellent Antebellum women's history, but has a rather unfair slant against the South. So take everything into context.
My self-published attempt is romantic historical fiction set in during the Civil War, but I put a great deal of time setting the scene and being accurate. Historical figures make appearances, etc. I try to visualize myself in the particular time and go from there. I actually was a Civil War re-enactor in college, so that helped tremendously in the visualization (plus I learned little details like split crotch drawers as Orianna2000 mentioned). I'm now writing in the 18th century and consequently decide to take up Revolutionary War re-enacting.
Something to consider though: the reason I'm self published is that after multiple requests for fulls and partials, the consensus from the agents was that the Civil War is not the popular time period in HF right now. For every Antebellum set novel you see published, you'll see ten European set historicals, and the Civil War era novels that are being published seem to feature real historical characters as protagonists. So if mass publication is your goal, you might want to investigate other time periods you find interesting.
Good luck!
That worries me ever so slightly--but isn't surprising. I noticed a pretty big difference between querying my French-Revolution WIP and my Antebellum South WIP. I'm still hoping an agent thinks it's worth taking on. I know the Civil War is pretty saturated but hope the Antebellum period is attractive for some of the reasons the Civil War is but interesting because it's slightly different.
Also, oldhouse, I've been thinking of getting involved in reenactment, but I don't really know where to get started!