Disasters are basically where something goes wrong with what your protagonist is trying to do--and it should happen all the time; or else everything will seem far too easy. No disaster=terrible plot.
In my eyes, there can be three kinds of disasters.
1)
The Annoying, Discomforting Disaster
This is one that often shows up early in the novel. And really quick, please note that disasters should get worse and worse as a novel wears on--there are few exceptions.
Here's a couple of examples, each affecting the next;
-The Protagonist scares the horse on accident, and the darn thing runs off with all of his supplies on its back.
-When the Protagonist goes and searches for the horse, he finds it--but without the supplies.
-The Protagonist searches for the supplies. It begins to rain.
-He finds the supplies, in the end--but they're soaked, and most of them are no longer edible.
Small disaster. It's not to hard to think up--but usually, it should have an effect on what happens next.
The key to many disasters is (to quote OYAN) to "give the Hero what he wants, but not the way he wants it." In this case, the Protagonist wanted to find the supplies--he found 'em all right, but not the way he wanted them.
2)
The Disastrous Loss
This is the second disaster. It usually consists of some sort of loss, as small as a family heirloom essential to the plot or as large as the loss of a friend. It can start out with something small, like the loss of supplies mentioned earlier, and then grow into something large.
Usually, the loss affects the plot in a significant way. The Annoying Disaster does too, but to a lesser extent--like chasing the horse made the Protagonist lose precious time, or have an enemy catch up to him. But loss causes a significant setback that normally makes the Protagonist have to change his plan in some way.
3)
The Ultimate Defeat
This one doesn't need to be described much, as it the title does it accurately; this disaster causes the Protagonist to be defeated. Most of the time this happens at the end of the book, the part where the Antagonist triumphs for a short while and has the upper hand. That, however, may or may not be in a novel, but if it isn't, consider if you should change that or not. If the Protagonist's goal is reached too easily, then you should probably make it a lot harder.
In some ways, I myself take a fiendish and rather strange delight in making up disasters for my protagonist. Really, it's fun! Dream up some scenes--and find out what it takes for everything that the Protagonist so carefully planned to go terribly wrong.
One more thing before I finish this; if you are having trouble with a part of the story that you know has conflict (which is one of the chief problems with failed novels; lack of conflict), it probably needs a disaster or two to make it harder for your Protagonist. :) Plot problems aside.
