Forward momentum in the short story is usually overlooked in books and articles about techniques. For me it's equally important as the other well known techniques. Because of the length of a short story it doesn’t take much to slow it down and and weaken your story.
1. Watch your proportions
In a story examine the “chunks” of dialogue and description. Large “chunks”
slow down the momentum.
Although dialogue is an excellent way to move the story ahead, too many long sections of dialogue can work against you. Dialogue is not narration.
Single lines of dialogue will speed up the pace and momentum of your story.
Anything longer than about half a page is too much. The exception would be a
Story told all in narrative passages. But again, the lengths of the paragraphs will determine the pace and momentum of your story.
2. Use of Flashbacks:
The use of flashbacks is to be discouraged when writing short fiction. Nothing
stops forward momentum better than a flashback.
A flashback like this is acceptable:
Harry remembered the last time he saw Julia and how beautiful she looked in the moonlight.
I have sometimes used a small tape recorder to record myself reading a story. It's a very good way to detect the forward momentum.