Take students on a field trip to a farmers' market and have them write about the experience in detail. Make sure students are describing clearly and using several words or phrases for one subject. A character's hair might be red, but is it silky? Curly? Straight? What color red -- is it dark, light or does it have a hint of brown in it? Details help form good content by giving it context.
Have students look for a human angle for their story to help readers relate better to the subject matter. Use interviewing techniques to gather content. In his book "On Writing Well" author and writing coach William Zinsser acknowledges that even science writing is more interesting when there's a human element. Have students relate their own experiences to the story.
Teach students to form an outline of their story to lay out - almost like a blueprint - of how they will tell the story. In his book "Writing Tools" Roy Peter Clark tells writers to "leave gold coins along the path" by the placing good solid parts of the story to hold the interest of the reader. Having students outline their story will help them to not give all of the good stuff away too soon. Remind them of their favorite movies and how the fulfillment of the story never comes until the end.