Weed out the Irrelevant.

Weed out the Irrelevant.
I have those words on a physics classroom chalkboard in a scene in my novel, "Love, Sex, and Understanding the Universe." Those words on the chalkboard are not important to the scene, but they are relevant, and those words are relevant to most any situation.
They certainly are important and relevant when it comes to writing.
In a writer's group I use to belong to, one of the other writers was reading a scene from her novel. At the beginning of the scene she has her character take his wool cap and gloves out of the car. She makes a point of showing us this action so I kept looking for the relevance in the scene. Does he leave them at the party and this is why he goes back the next day and meets the person who is going to change his life? Does he turn out to be the only one who is prepared for the cold night and thus impresses his date? Does it turn out the cap and gloves belong to someone else thus revealing him to be a thief? Do we see this behavior so that we'll know he's the kind of guy who's always prepared? No. In fact, the hat and gloves are never mentioned again, and when I asked why she put that in there, she said she just wanted to keep the story true to life by including what a person would likely be doing at that moment.
We can't keep our fiction TOO true to life. There are so many details in life that are... well, irrelevant. And these details, like the cap and gloves - become distractions from your story. Likewise, if you included every hello and good-bye for every conversation in your book, it would drive your readers to boredom. But there may be times when the particulars of the encounter make including those necessary or important.  Sometimes the relevance of a detail is subtle but very useful - as in my chalkboard words.