One of the trickiest issues to manage when writing historical fiction is the urge to "overpack." You have all this research, all these amazing details and factoids and lovely little tidbits of information about what life was like back then (whenever "then" is), and you want to share them all. At once. En masse.
Let's put aside for a moment the question of motivation. After all, does it matter whether you're trying to put in all those details because you think it's necessary to create the proper atmosphere, or because you feel a need to prove that you know your stuff? (In my case, it's often out of a fear that sometime, after the book is published, I'm going to get a call or e-mail from a graduate student somewhere in, say, Bulgaria, or Germany, or Dubai, or deepest darkest Berkeley, to protest that so-and-so had already died three years before I had him appear in my book, or that stirrups were made of wood, not metal, at that period, or whatever.) The more important question is this: does that level of detail really belong there?
Remember: we're writing historical fiction. Not a doctoral dissertation. The critical requirement is that you create an effective story that immerses the reader in your fictional world, and keeps her hooked on your unfolding narrative. It has to feel real. And when you're trying to make something feel real, sometimes less is more.
Many craft books on writing talk about the "iceberg principle" - the idea that behind your narrative's surface, there should be a vast amount of life and thought and emotion and "story" that is hinted at, but never directly shown. You need to give the reader a chance to put her own imagination to work, filling in the texture and smell and context around the latticework of (limited) specificity that you explicitly describe. Invite the reader in. Don't bludgeon her with information.
The thing to look for, as you do your research and plan your work, is not how to amass a complete, seamless, irreproachable description of a given historical place, event, or person. You're not trying to get it exactly right on the surface. You're trying to make it feel right - and often the way to do that is to tell less, but make sure the details you do provide are so evocative, so attuned to the spirit of the whole, that they will suggest a whole world around them.
You're looking for the "telling detail."
It can be a smell - that precise odor of animal fat and grease and soot of an unperfumed tallow candle, for example, or the mildewy mustiness overlaid with creaky finger-stained leather of a vellum codex. It can be a sound - the arhythmic slap-kerslap of a sandal with one sole flapping half-loose. Or a personal quirk that, historically accurate or not, suggests an entire personality that will bring a figure to life, like Marie Antoinette's predilection for pretending she was a simple shepherdess, keeping a little herd in the midst of the elaborately manicured gardens of the palace, maintained by battalions of servants. Whatever it is, the detail is the tip of the iceberg that suggests the enormous mass lying beneath. Whether or not you truly have researched and thought through everything that supports that "telling detail," your reader needs to believe that you've got it all in your head.
Think of it as though you are filming a movie on a bare-bones budget. You can't fly your cast to, say, Numidia and spend months shooting on location. You need a backdrop that will make the audience believe, utterly believe, that that's actually where they are. How are you going to pull it off? Pretend you're filming Casablanca. Humphrey Bogart and Ingrid Bergman and Co. never got anywhere near Morocco, yet somehow we never doubt - during the time-suspended period that we're watching the film - that we are there, in German-occupied North Africa, in 1941. How did Michael Curtiz manage to take a recycled set on a Warners backlot and conjure an entire world out of it?
As you're working on your historical writing, look for "telling details" that you can interject to conjure your world, and then look for the natural places to put them, as though you're planting Pandora's boxes in the underbrush for your reader to trip open at key points. This, I think, is one of the ways you'll keep the magic alive.