Nothing can be there for “just background”. Nothing can be peripheral. Nothing can be “filler”, just to stretch out the pacing. You don’t get much in flashbacks or other developmental “side trips”; you show a character’s history as s/he reacts to the other story elements.

I recommend you hit your local library for a book on writing, especially if there’s one specific to short stories.

For instance, if you start out wanting to write the greatest story about divorce ever, it will be hard to accomplish the goal in a short story. Instead, you should pick a scene from a divorce. Maybe it’s the child’s birthday and the parents are arguing but the short story is only that ONE scene. The scene may end up having larger implications on divorce or abandonment or wherever else the story leads but those will come across naturally. In other words, themes should come after you write, not before.

That brings me to another point. Try to slow down the passage of time. A short story could (and likely should) cover brief moments in time.

This might seem challenging, but the point is that you are showing enough detail and action and making every word of that scene count enough that it doesn’t need to take up hours of time.

You’re showing, not telling. If you use the divorce story as an example, think of it this way. Telling might be:

“His parents were screaming at one another and Jimmy felt scared.”

Telling might be:

“‘You lied to me!’ his mother yelled back to his father. She had that look on her face, like when Jimmy drew on the walls last Tuesday.

His father said something loud back, but Jimmy didn’t know what it meant. In his head, he counted the blue-and-white striped candles on the cake: One, two, three, four. They were next to the wax spaceshuttle that said ‘Moon Mission.’ Did they yell on the moon too? The cake seemed to blur once his eyes filled with tears.”

See the difference? Showing what happened fleshed out the story. Telling what happens is giving a framework, but without the important details. The better the level of detail, the better the story.

So those are my ideas. Good luck!