Your Ending is the point where the struggle your protagonist has battled is finally resolved.
As you lead up to your ending, your sentences should become shorter; paragraphs, shorter. You are speeding the reader up as you race to the conclusion. No lingering.
The most common mistake most beginning authors make is not ending the story shortly after the resolution.
Depending on the genre of the story, you may have an anticlimax to wrap up loose ends, BUT KEEP IT BRIEF.
Another common problem is not resolving the conflict. The reader has read your tale and expects the protagonist to resolve the primary conflict. The author who fails to provide a satisfactory ending, a believable resolution to the conflict irritates his reader.
On the flip side, if your ending is too ‘neat’, it will feel contrived. It loses its believability. For example, when the wagons are surrounded and the bullets are gone, the Indians are closing in and then the Calvary shows up accidentally over the rise of the hill, pfft. Out of all the places they might have been, they just happened upon the wagons? NOT! Now if earlier a boy had been out hunting and seen the problem and ridden for help, bringing the Calvary back, now its believable. Remember your cause and effect of the plot must carry through to the ending.
Your ending should be the probable outcome of the chain of events you have strewn across your readers path. When the reader thinks back over the plot, it should appear to be the reasonable outcome of the events.
That is not to say the expected outcome. Readers appreciate an unexpected ending. When it appears, it must seem natural, it had to happen, but it should not be something the reader anticipated. If the reader knows the outcome, why bother reading the book? On the other hand, a ridiculous contrived surprise out of nowhere is not believable and will also disappoint.
No ending is too insane to use as long as you set it up and have prepared your reader for it. Think Sixth Sense. If it is not set up, you lose them. If it is set up, you will have them looking for your next novel to hit the shelves.