What's one general rule I can talk about that's at the heart of writing your story?
I came up with this simple statement: if it isn't believable, it doesn't belong in your story. Dump it.
Doesn't matter what kind of story you've written, how far out, how close to home, how extreme the world you've created, your reader comes to that story wanting to believe it. They're not suspending disbelief, they're embracing yours.
Your reader doesn't want to consciously set aside his or her belief, but rather wants to engage in the story you've created, live in the world of your making and believe in everything you, as the writer have created to make your world real.
With readers ready, eager even, to immerse themselves in deep belief in what you've created your goal is to keep their belief alive throughout the story.
So, consider the world you've created. What are the parameters that define it?
For example, you've created a world that's made up entirely of water. There's no land - anywhere. You've brought that world to life and your reader accepts that world as being very real - but now you have to remain very consistent. The people who populate your world, living on rafts or in bubbles or above the waves in cloud homes will have to deal with that world as you've created it. If you suddenly toss in something to make your story a bit easier the consistency is shattered.
If you break the consistency, if you suddenly throw in a chain of islands or change directions and suddenly make it known there was land there all the time, your reader is going to be caught short, probably begin to lose interest and most likely stop reading.
Or, worse, he may become distracted from your story by beginning to look for inconsistencies you've created throughout the book. Not good. Not good at all.
But another problem that can arrise lies with your character. If you've kept your world consistent, don't mess it up by allowing your characters to act in unbelievable ways.
Just as your readers are involved in the background, the world of your story, they're greatly interested in the characters. The characters must act naturally, to do what they would do naturally in a situation.
If you force your characters into situations acting in ways that aren't natural to them and believable, either in the way their character was created or to how the story is moving forward, the reader is again jerked off track. Do this enough and the reader simply puts the book down.
But, you say, there are times when something odd will happen in a book, or a character may behave strangely. Very true. But if that is the case then let your characters notice it. Respond to it. Whether a character responds in his thinking or in action or dialog, let the reader in on what's going on. A simple statement like "I didn't expect that!" or "There's more here than what the first rain revealed," might do the trick. If you do, rather than confusing and disengaging your reader, you'll draw them even tighter into your story.
If your character behaves in some way unbelievably give him a good reason, a really good reason, and you'll give your reader even more reason to read on.