Do not anticipate your climax, and get there too soon, and then try to make up the book to the required length by adding on an after-piece.
The climax should be such that it leaves in the reader's mind a sense of absolute fitness, a certainty that it was after all the one right ending--even though it came as a great surprise.
When a story is presented in sections, as in a serial or a play, it is advisable to make each section end--so far as possible--in such a manner that the reader is set longing for the next part. Thus, while the climax is generally the solution of a problem, a "curtain" is usually a problem needing solution (literally, a good place for ringing down the curtain, since the audience will be on tenterhooks to know what happens next).
This arrangement is sound business as well as a good mental policy. It is wise to make an instalment leave some final, incisive mark on the mind of the readers, if there is to be an interval before the story is resumed, otherwise it may be difficult for the public to recollect what went before, and the thread of continuity will be lost.
More than this, an editor, despite the usual backwardness of his intelligence, realises the desirability of securing readers for subsequent issues of his periodical, no less than for the current number. If each instalment of the serial terminate with some mystery unsolved, or some hopeless entanglement needing to be straightened out, or some problem that baffles everybody (most of all the readers), it is much more likely that people will rush to secure the next number to see how things turn out, than if the instalment merely ends with the hero indulging in a tame, lengthy soliloquy on artichokes, and leaves nothing more exciting to be settled than whether these same artichokes shall, or shall not, be cooked for the heroine's lunch.
On more than one occasion I have had readers write protestingly because an instalment of a serial has left off cruelly "just when one was frightfully anxious to know what would happen next!" But that is the very place for an instalment to end: good "curtains" are worth as much to a serial as a good plot; and if a story lack good "curtains," an editor thinks twice before purchasing it for serial publication, even though it has undoubted literary merit and will make a good volume.