Maintaining suspension of disbelief is one of the most vital (and intangible) elements of a work of fiction. The ability to carry your reader with you, through many plot twists and turns, is something that marks out the truly talented writer from the merely skilled. But it's also very personal – one reader may stay glued to the page from start to end, but another will suddenly break off mid-way through chapter ten, claiming that the whole thing is just too ridiculous.
There are broad things we can do to encourage maximum absorption: smooth writing that flows and doesn't draw attention to itself, believable characters, and a plot that builds rather than changing direction on a whim. But much depends on tone and confidence. It's my belief that a strong voice always comes from a writer who is confident about their material and prepared to take charge of the narrative. A reader can gauge your level of confidence from the very first page – they will quickly decide whether they trust you to guide them through the story world and reach an appropriate ending.
I had a big argument with another writer at critique recently, about whether an author needs to understand why or how things function in their fictional world. I said (rather too stridently) that a writer should understand how everything works, especially in fantasy worlds – even if they don't describe these mechanisms to the reader. The other author was unable to see why this was a problem – couldn't I just accept that these things worked as described and move on? But for me, that lack of knowledge showed up on the page and shook my belief in the world and its rules.
It might be my engineering background or basic control-freakery, but I have to understand all the whys and wherefores of everything that happens in my books. The idea of getting it right is very important to me and hugely disruptive to first drafts, because I spend more time thinking about the book than actually writing it. But when I don't know how something works, it just niggles at me until I fix it. Clearly I wouldn't be a good candidate for writing magical realism!
Character motivations are another area that is important for maintaining suspension of disbelief. If a writer understands their characters' motivations, then they are able to structure the plot appropriately. Using characters to drive the plot is a basic tenet of story theory, because it keeps the narrative believable, even when the events are unlikely. If the reader can always see why something happened and why a character acted a certain way, they will be far more accepting of the outcome.
I've lost my ability to suspend disbelief during quite a few novels, when a single scene stretched credulity too far – often to cries of "Oh, come on!" As a writer, I'm probably more demanding in these areas, but it's true that once a reader loses suspension of disbelief, they will suddenly start to notice everything else that's wrong with your story. When that happens, you'll have to work twice as hard to win them back.