Know Your History!

Something fantasy and science fiction writers struggle with more than writers of other genre's is their history.  The history of a race.  The history of a world-or a galaxy for sci-fi.  The history of a person.  There's just a lot of history that goes into these two genres.
But what about one of the biggest rules of writing?  Show, don't tell-though I myself think rule, like all in writing, is meant to be broken.  We can't quite show the history can we?  Without doing flashbacks or just making our characters read a history book from the library, no, not really.  And both of these are, in themselves telling--tip #45--though they don't sound like it at first.
There is also the dummy approach in which you pretty much have your protagonist or another character be a dummy and have another character inform them on the history of something.  This can be used for explaining pretty much everything you deem that the reader needs to know.  But you have to be careful, too much of this at one time can lead to telling by itself.
However, often you don't even need to tell your readers the history.  This is where knowing your history comes in.
Many times, when you write the history of something in your novel your doing it for yourself.  You want to know what happened before your story takes place.  As well you should.  But don't put it in your book.  I would urge you to write the history in a notebook or on another file.  It might be helpful to just flatout tell the history like a history book would.  But don't publish it in your book.  If you know your history then it will come out gradually in your story.
That notebook or that file with the history may even some day become published depending on how well your first novels went.  Consider the Silmarillion by Tolkien.  Granted, he wrote the Silmarillion after the Hobbit and Lord of the Rings because his fans demanded it.  But the history wasn't in the novels.  One of the great things about Tolkien's books was the history.  But if you think about it, it wasn't in his main novels such as LotR and the Hobbit.  It was a side note, for people wanting to get even more of the background knowledge of his world.  And if you've read the Silmarillion you know that it's as boring as anything.  But it wasn't meant to be a page turner, it was meant to explain the background of everything.  I haven't had the chance to read the Histories of Middle Earth, but those are, I believe, about seven volumes of the little details, even more historically written then the Silmarillion. 
Anyway, back on topic.  Know your history.  When you know your history and can look at it, it comes out naturally.  It doesn't seem forceful, like your shoving it down your readers throat.  It comes out gradually and your readers don't even notice they're learning about it.  You don't force it down them because you already know it, and part of the reason you write the history forcefully is because you want to know about it.