Before you tell a story, ask yourself “Why would my audience want to hear this story?”
Is it because the story is funny? Exciting? Touching? Interesting? What emotional response is your story likely to elicit from your listeners? (If you can't come up with an answer to this question, that might be a sign you should find a better topic for your story!)
The reason you ask this question is so that you can choose your details carefully, to make sure they support this goal. If you're at the grocery store preparing to bake a cake, you don't load your shopping cart with fish and jalapenos, because those ingredients don't help you bake a cake. Similarly, you want to pack your story with only details that support the goal of the story.
In other words, if you are telling a funny story, get to the funny stuff as soon as possible – don't waste time giving boring details. If you are telling a touching story, your goal is to tug on your audience's heartstrings, so be more careful when using humorous details. If you're trying to make an exciting story, then your goal should be to build momentum and suspense, so you shouldn't include details that sacrifice momentum.
For example: Let's say I want to tell a funny story about an experience in Mexico. Here are two versions – see which one you like better.
“When I was in Mexico, I decided to go to a club with some friends. We first met up at someone's house, and we spent some time sitting around and talking. Then, we walked to the club, but decided we were hungry, so we stopped on the way at a Mexico 7-11 and bought some Cheetos. Except in Mexico, they call them “Chetos” for some reason. After we ate our Cheetos, we went into the club. It was really loud but everyone seemed like they were having fun. We danced for awhile, and then I noticed that a guy in a bear costume came onto the dance floor. He started a conga line, and a bunch of people went on the conga line after him. They congaed right off the dance floor and into another room and I never saw any of them again. To this day, I wonder if the bear costume guy was secretly a kidnapper, and he lured the group away to abduct them!”
“When I was in Mexico, I decided to go to a club with some friends. I was dancing, and then I looked up and I saw a guy in a bear costume! He started a conga line, and a bunch of people joined up and congaed right off the dance floor with him, into another room. I never saw any of them again, and to this day I wonder if the bear costume guy was secretly a kidnapper, who lured the dancers away to abduct them.”
Most likely, you liked version two better. Why? Because it got to the punchline – the dancing kidnapper bear – much faster! All of the details about meeting at the friends house, the Cheetos, etc were all true – but they were irrelevant.
The point was to get to the punchline, so any details that did not support that punchline should be removed. In the first story, by the time I actually got to the punchline, you were probably skimming and wondering “What's the point?” So when the punchline actually arrived, you were not very invested in my story and you didn't find it very funny.
If you do this right, your conclusion will feel very satisfying to your audience, because everything in the story was building to that conclusion. Not only do you maintain their interest as you're telling the story, but you also create the possibility for a really powerful conclusion.