1.  A scene is the smallest unit of a story, the way the atom is the smallest unit of matter. It's not the word or the sentence, because in a scene you have the smallest basic element as the whole story, which is to say that in a scene, SOMETHING MUST CHANGE. You start somewhere and end up somewhere else. There is a tiny arc. Something changes. Just like in a story. 
  2.  An effective scene ends with a set up for the next one. A decision is reached. A risk is taken. The reader must turn the page to find out what happens next. It's how you develop narrative drive. A scene that just sits there and doesn't lead to anything? It's deadly to your story. 

  3.  A scene generally happens in one location. It's useful to think of bringing actors onto a stage. They do what they do, and then the scene ends and they walk off stage. 

  4.  Use dialogue to zoom in on elements you want the reader to pay close attention to. And remember that characters don't tell each other things they already know. 

  5.  Always let the reader know what the protagonist is thinking, feeling, and doing. They are in their skin, feeling what they feel.  

  6.  When considering what to leave in and what to take out, focus on why. Lisa Cron talks about this so well that it's worth turning to her words:

Concentrate on the “why” and not the “what.” Whenever something is about to happen, ask yourself, “why?” Why is this happening now? Why is my protagonist reacting the way she does? Why does the reader need to know this? Stories aren’t about “what” happens, they’re about “why.” Just like life. Watch as your day unfolds. People do things – that’s the what – but aren’t you always wondering why they did it, what they really mean by it, so you can figure out what the heck you should do in response? In a story the most important initial “why” is why the protagonist wants what she wants, and why she can’t seem to get it. Figure it out first, and it will be your true north. I would add that you should then keep thinking about why. You can get a very long way down the road of a story by asking yourself, "Why?" Why did the character do that? Why did she say that? Why does she care about that? Why would the reader care?