maybe you could do a plan before-hand for example, a mind map.
you could generate a random word and link whatever thoughts/feelings/colours/experiences etc. that they make you think of, then gather them into a story.
or, take inspiration from music, art or films.
when writing, start with a grabbing intro, then go into a descriptive plot, and finally a ending (this could also be a cliff hanger) quality and quantity of writing is important, also remember to paragraph.
remember to use a wide range of grammar, techniques and vocabulary for the best grades (this could be emotive language, ellipsis, alliteration, red herrings etc.)
hope this helps :)