1) Don't start your fantasy book in omniscient. For the love of God, don't do it. It may have been a good idea- once. By now, it's frankly boring to open a book and find it goes like this:
The Castle of Petronea stood tall and proud on cliffs overlooking the Castinian Sea. The cliffs shone white in the sunlight, and sometimes the seabirds that nested in them dropped feathers that the children found and used in their games, all ignorant of where they came from. On the western wall of the castle, one could look out into the sunset over Petronea's fair forests. On the eastern wall, one could look down into the sea.
No connection with the characters, no way of telling who's saying this, no way of telling where it's going. And, usually, at least three more pages, or long paragraphs, talking about the room decorations, the gardens, the royal wing, the stables, the mages' quarters, the history of the castle, and anything else that you can think of me as skimming with glazed eyes to get to the real story.
You know that saying how character= story? Or, for that matter, how plot= story? Well, whichever one you believe, why are you starting your book this way? There are no signs of characters, since things like "one" and "children" don't encourage readers to identify like names and personalities and dialogue, sweet underappreciated way of beginning a book, do. There is no sign of plot. This could be a romantic fantasy, a political intrigue-driven one, your typical feisty-princess-who-doesn't-want-to-be-a-l
Go away, and come back when you have a real story.
2) Stop jumping in and out of omniscient and third-person-limited. It's stupid. Once again, countless fantasies will do this. We'll have several scenes, or even a full chapter, in one character's head. Then we'll get this:
Enrenda nodded. She couldn't believe that her parents were trusting her to take care of herself for three weeks on a long journey alone, but who was she to argue?
There were robbers in the hills who would kill and rape many young girls. Enrenda didn't know it, but there had been three hundred girls raped and killed in the last year alone. The king was trying to do something about it, but even he couldn't do much when the robbers killed all his messengers. There were heads scratched in the palace day and night, and still no one could come up with a solution to the problem.
"What are you going to take on your trip, Enrenda?"
Enrenda smiled at her mother. "I was thinking a riding skirt..."
This is a particularly virulent piece of stupidity because the author takes care to note that Enrenda doesn't know the exact numbers of girls raped and killed, thus snapping the pretense of her viewpoint completely.
Stop it. Your reader might not notice the viewpoint shift- although I can guarantee you that many careful readers will- but it is your job as the author not to be careless. If you intend a third-person limited character, then you have to accept that there are things a third-person limited character cannot do or think about. Find a different way of introducing the information into the story than violating viewpoint limitations whenever you feel like it. It's lazy and imbecilic.
3) When switching viewpoints in the same scene, Eru damn you, don't just jump. There are natural ways of leading into this, such as following a literal line of sight. You can have one character glancing out a window at a second character standing in the garden, and the second one looking up and returning her gaze. Then the viewpoint can flow naturally along to the second character: "As Rendorn looked into Ariana's eyes, he wondered what she was thinking." The same thing can happen with mirrors, or even just across a room. It's smooth, gives the reader something to follow along with, and maintains that all-important illusion of the story being alive on its own, not just something the author is writing down.
Other ways of doing this that are less smooth are having one thought reflected in another character's mind, the scene staying with one partner in a conversation while the other moves away, and the story shifting to follow the person who's doing something important while the one who brought you there sleeps or drops unconscious or whatever. I don't favor them as much because I think it, again, seems more noticeable= more artificial= snapping the spell. People get distracted enough while they're reading, by interruptions and random thoughts. The author shouldn't embed distractions in her own work.
On the other hand, I also don't shift viewpoints in a scene; mine always come at chapter breaks. So perhaps there are writers who can make these work better than the line-of-sight thing.
4) Make sure the reader can tell why you chose that particular viewpoint. This should be obvious at least halfway through the story, if not earlier. There's a plethora of reasons, but I cry if I can't sense any at all. (Well, not really. I seethe. Then I call the author an idiot. Then I come on here and rant).
I've seen dozens of stories that would have worked much better through a different character's eyes, especially because they were the ones doing the most exciting/best-written/most interesting things. Now, if the author is writing about a main character who cannot tell his or her own story for whatever reason- like mental retardation, lack of understanding, knows everything and would give the game away, etc.- I consider that a plausible reason for the author to write that character's story through the eyes of an outside observer. But when the narrator is obviously meant to be the main character and yet is dull as dishwater next to the shining "sidekick," something is wrong here.
Likewise with first-person narrators where the author takes the opportunity to slip into omniscient, "Well, I learned this later" bullshit. Does the story really work as a first-person story? If not, give it up.
Too many authors resort to the omniscient because without it they would not have a clue how to plot. Learn to plot. Yes, it's more work. Good. It will cure that laziness.
Too many stories make me want to reach for seasick medication lately. They need trimming, or even the axe that the firemen used to chop through my neighbor's door.