OK, so your world doesn't have to follow real logic. You can have talking beavers and faster-than-light travel, and whatever else your illogical heart might desire. But your world should follow its own internal logic. Set the rules of your world, and then only break them on special occasions. For example, the universe of The Hitchhiker's Guide to the Galaxy isn't logical by our standards. There are living mattresses and two-headed presidents. But Douglas Adams follows the rules of his own nonsense world, in which Vogons are always bound by bureaucracy and towels are always helpful (and you still can't breathe in outer space).

You don't have to put a map in the front of your fantasy novel. But you do have to know how big your world is, and where everything is in relation to each other (also you should definitely put a map in the front, because map's rule). Even if it's just for your reference point, sketch out a few basic maps of your world and your chief locations, and make sure distances and travel times stay consistent.

I'm sorry but... in worlds like Westeros or Narnia, where winters can apparently last for decades, how do people eat? Do they just store enough grain to last ten years? Think about the boring basics of your world, like how people eat, how the economy works, and who (if anyone) handles garbage pick-up. You might find that the boring questions lead to more interesting questions... like if there are no garbage men in this world, do they have advanced recycling? Do they have rivers of sewage? Do they feed their garbage to a race of underhumans who live under the streets?

Once you've decided on the broad strokes, boring facts of reality, you can start adding little touches that make the world feel real, like regional dishes, popular pet names, or favorite rock bands in this universe. Not all of these details will make it into the final draft, but they add dimension. I mean, the Weird Sisters band isn't essential to the Harry Potter universe, but it feels like a realistic touch. And who is that wizard reading A Brief History of Time at the Leaky Cauldron?

You can't condense all of a world's history into a handful of novels (George R.R. Martin is trying to do that, and it's taking him forever). But sit down and decide what a history class on your fictional world would look like. Which wars make it into the syllabus? Who are the major figures? Who's left out of the history book?

You know how in Star Wars it seems like every planet just has one major biome? Yeah... don't do that with your world. Unless you're writing a creepy dystopia where everyone is exactly the same, remember that even the most homogeneous of societies have some level of diversity when it comes to different opinions, religious sects, nationalities, plant life, etc. Be careful not to write monolithic cultures. There is no culture on Earth where everyone is identical and agrees on everything, so build every kind of diversity into your fictional world, too. Also... remember that you are making the rules for this world, so there's no reason that it has to have all the same limits as our real world when it comes to the roles of women or racial minorities.

Because really, world-building is all about balance. You, the author, should know all the juicy secrets and tedious governmental structures behind your world. And your reader should know just enough that the world feels textured and fully populated, without feeling overwhelmed by your detailed descriptions of every different family's crest. The only way to strike that balance is to learn by example, so read your favorite fantasy and sci-fi authors, and note where and how they slowly unveil that inner workings of their worlds.

...and then put a unique spin on it. Most great world-builders draw from some aspect of the real world (usually from medieval Europe), and then magic it up a bit. Borrowing from real governments, countries, and ecosystems will help make your world a little more grounded. Don't limit yourself to European history, though: maybe your cat inspires you to create a race of furry little wizards, or maybe an abandoned house in your town makes you think of an entire city of the dead.

Specifically, don't create a one dimensional "fictional version" of a real world culture. Nothing ruins a story faster than if your cool aliens are really just a lazy ethnic stereotype, but green. It's fine to be influenced by real traditions and histories, but try to recognize when you're using your in-depth research as a jumping off point, and when you're skating by on cultural cliches... just basically read J.K. Rowling's History of Magic in North America and try to do the opposite of that.

Be the obnoxious, nerdy fan of your own story. Keep asking yourself questions like, "What are the social ramifications of purple-eyed sorcerers who live forever?" or "Why do I automatically feel like the more active roles in this society should be men?" or "Wait, shouldn't the Citadel of Darkness actually be two miles west of that location?" The nitty gritty of world-building might get tedious sometimes, but when you step back and admire the beautiful tapestry of fiction you've created, you'll be glad you took the extra time.