1. Use only two main characters, one male and one female (such as one "he" and one "she", so there is no possibility of ambiguity). Try developing a couple of characters you enjoy writing about in advance, and practise grafting them into a variety of story scenarios.
2. Plan your "Beginning" (characters, setting, some sort of problem) and "End" (resolution). Keep it simple.
Storylines don't need to be amazingly original - it's the way you tell 'em that counts.
3. When you know your ending, find a way of feeding something about it into your beginning. This contributes to a good "resolution".
4. Keep the beginning short. The main part of a story is the "Middle" (how you get from the problem to the resolution).
Get your story moving as soon as possible.
5. Keep your reader informed about any changes in the setting as the story goes on. You may be seeing it in your mind's eye, but your reader needs to be told what is happening.
6. Similarly, keep the reader up to date on how characters look and feel as the story progresses.
7. Use direct speech to help carry the story. Make characters talk about what's happening. Always include a reporting clause (such as "replied Peter") but don't always use "said".
8. Use a variety of sentence lengths, and avoid repetition, especially of sentence openings. If you find every sentence starts with "They" (or even worse "Then") look for another way of expressing yourself.
9. Find a neat, punchy line to end on. Last lines are important.
10. Aim to finish with five minutes to spare, and read your story through to see if you can improve it (perhaps by adding description, or changing a weak verb like "went" to a more powerful one). Pretend you've never read the story before, and check where more detail is needed to bring it to life.
11. At the end (and at any time when your concentration flags or you need a break from writing) read your story through aloud (under your breath), checking punctuation. Reading aloud helps you decide when you need a full stop, comma, or other grammar. Pay special attention to punctuation of direct speech.
12. Finally, check for spelling mistakes by looking at the words but not reading them! If any word glares at you as obviously wrong, try it a couple of ways on your planning sheet. Make any corrections you can.