Having run Short Story competitions for the past few years I feel I am somewhat qualified to guide eager story writers along the path to prizes by avoiding those major pot holes that so many fall into.
1. Don't Read The Rules. As soon as I take a story out of its envelope I can tell if the writer has read The Rules or not. If the pages are not numbered when I've said number them; if the story title is not at the top of each page when I've said do it that way; if the text is not double spaced or the font size needs either a magnifying glass or has to be read from the far side of the room I know they haven't read The Rules.
Competition organisers say how they want your manuscript to look for a reason - usually it's so that the story can be read comfortably, but also so that if the judge should throw a batch of stories up into the air in sheer exasperated frustration the resultant mess can be sorted out reasonably easily if The Rules have been followed.
And another thing: not following The Rules gives a bad impression to the judge and, depending on how the organisers go about rating your story, he (the judge) may well mark you down for it. Don't fall into the trap of thinking "My story is so good they'll ignore little things like font size and so on". Don't you believe it! It's even possible that some mean minded judges will put an entry straight into the bin if The Rules have not been followed to the letter.
Lesson: If you don't want to win the competition - ignore The Rules.
2. Don't make your story a story, instead make your entry an article or a monologue. What's the difference? Simple. A story unfolds in real time with dialogue, emotion and description, whereas an article recounts a series of facts by an impartial observer, and a monologue is one person talking about their experience(s). Which is not the same as a story.
If your story sounds like an excerpt from your personal journal, it's an article.
Here's one definitions of a monologue: "A monologue is an extended uninterrupted speech by a character in a drama." Does your story read like that? It's a monologue.
When I say that "a story unfolds in real time" I'm not saying it should be written in the present tense. It may well be, if you have good reason to do so, but either way the reader, in our case the judge, should be told the story in proper story-telling fashion. Remember you are trying to win a prize and unless the competition organisers have asked for something more cerebral, then stick to a true story format. Here's Wikipedia's definition: "A short story is a work of fiction that is usually written in prose, often in narrative format."
Lesson: If you don't want to win the competition - send in an article or monologue rather than a story.
3. Don't stick to the theme. If the competition asks you to write a story around a particular theme, then stick to it. Make it clear shortly into the story that you have embraced the theme. Sticking it in right at the end as if it's an afterthought may not impress the judge. In my experience some entries appear to be stories that have come out of the archive drawer regardless of theme, bunged in an envelope and sent off. Job done.
Lesson: If you don't want to win the competition - ignore the specified theme.
4. Don't use the spell checker or have your story read by a friend who can point out spelling errors. The thing to think about here is that the competition organisers may well want to include winning stories in an anthology or maybe publish them on their website. To use a story that has spelling mistakes, bad grammar, words missed out, words duplicated and so on is not going to do their street cred any favours. And remember it's not their job to proof read and copy edit submissions. The author is supposed to do that.
Lesson: If you don't want to win the competition don't get it spell checked or read by a friend.
One final point. There are some 'competitions' that will accept any old rubbish and publish it, so long as they can get your entry fee, and maybe persuade you to buy some copies of the anthology as well. It may even be that the 'prize' IS publication in an anthology. Take care that the competition you are entering is genuinely looking for quality material.