Someone very wise (whose name escapes me) said the beginning is what sells a story to the reader but the ending is what sells your NEXT story to them. If the ending isn't satisfying, then the dissatisfaction the reader is left with will probably be all they remember.
So...how can we make an ending satisfying? Here are some ideas I try to use. No doubt there are others.
- Something changes. A story where nothing changes from beginning to end leaves a reader thinking, "What was the point?" The ending should show that there's been a growth or progression in some form or another. This could be a change in the character's beliefs, actions, situation, or personality. It could be a revelation of something the reader didn't know about before.
- Something surprises. Put in a twist that the reader doesn't see coming. Ideally, it is also something that makes perfect sense so it doesn't feel artificial. Some of the best twist or surprise endings let the reader go back through the story and think "aaah" to themselves as they see the way it was set up ahead of time, now that they know. The movie "Sixth Sense" is a good example of this.
- Something impacts. An ending can make a reader feel emotion. The earlier parts of the story should have made them like and identify with the characters and the situation - here's where you can take advantage of that. Twist the knife. Or bring the lovers back together. Whatever emotion it is, make them feel it and they will love you for it. Or, at the very least, your story will stand out.
- Something circles back. One technique is to let the story do a full circle, taking the reader back to the images and events of the beginning. This may serve to highlight what has changed in the space between, or to make the reader feel the emotions related to the fact that the characters have gone through an experience but are back where they started. Either way, this can give a sense of satisfying symmetry to the story.
- Stop when you're finished. This might sound obvious, but often writers feel they need to explain the story events or tie up all the loose ends. Trust your readers. They're not stupid. Let them draw the right conclusions by giving them enough to do so without spoon feeding them. A long wind down at the end of a story - especially a short story - is usually unnecessary and often dull.