We writers all know that not all scenes in your story are created equal. Some scenes we spend weeks preparing for; some scenes we feel afraid to write; some scenes we are wildly excited about finally, finally getting to. Those scenes are the keystone scenes. If you're familiar with your ancient and medieval architecture, you'll know the term: it refers to the center stone in an arch, the one that bears all the pressure of the arch stones and keeps the arch together. The keystone scene in your story is the one (or two, or more) that holds the story together and makes the whole piece of writing "worth it."
If you're going to write a story you can be proud of, you've got to make sure the keystone scene is your best. Understanding the structure of your story and what needs to happen will help you understand what kind of scene your keystone needs to be.
Choose your keystone.
The first step to making a great keystone is actually figuring out your keystone actually is. One way to do this is to read other stories and identify the crucial scene. The keystone scene is the one where the crucial change takes place; it's when the character has a choice to make. The writing will often become more detailed and will slow down in anticipation of this momentous event. And while many things can happen in a glide (a summary rather than a real-time scene), the keystone is almost always a scene, unfolding right before our eyes.
With that in mind, look at the general trajectory you've planned for your story and figure out what moment the keystone is. Is it when your character leaves his family, or when he dramatically returns? Is it when he finally tells her what he thinks of her, or is it when she responds? Choose the scene upon which all the other scenes depend, the one that has a sharp feeling of before and after.
Buffer your keystone correctly.
There are certain clues a story gives us that tell us to anticipate the keystone scene. Without that sense of rising action, building tension, and intensified drama, we would never know that this is it, the moment we've been waiting for. The crucial scene can often be buffered on either side by brisk movement through time in glides. When we drop into real time, then we'll be aware that this is the first important stuff in a while. Other ways to keep the keystone strongly defined is to raise the stakes immediately beforehand. Make it clear that a choice will have to be made, and the consequences of that choice have now become much more serious.